As already has been said, Photoshop is an extremely complicated program. SO! Let me give you a few tips on your questions above, then I'll give you a few links to hundreds of tutorials, so you can experiment, and have FUN! (S'what Photoshop's all about, right?)
Make yourself look skinnier:
1.With Photoshop open, bring your image out into the workspace, (I'll just refer to a photo, or photo of you as IMAGE from now on.) Go up to the top header,(File, Edit, Image, Layer, etc.), and click on Filter. In the Drop-Down menu you will see Liquify. Click on it. Now if your image is, let's say, at 25%, and you can't see it as well as you'd like, go down to the bottom left where it say's 25%,(whatever size is listed. This is just an example.), and click on the little Down Arrow next to it,(sorry about the run-on!) This will give you various sizes to choose from. This is just to view the image, it doesn't actually change the size. Okay, better? Now go to the Tools Menu on the left, and click on the Pucker Tool.(Hey, I didn't make these names! lol!) (Also, you may not be aware, but you can 'hover' your mouse cursor over a tool in the Tool Menu, for a cupla' seconds, and the name of the tool will reveal itself!) Go to the right, and click on the Side Arrow next to the Brush Size, under Tool Options. You may want to make the Pucker Tool larger, or smaller. It's all experimentation, at this point,(so use it, then change the size, and try it again). When you click on the side arrow you will get a drop down slider. You can click n' drag the arrow on the slider, or just click on the line, to make the arrow move to that spot. To the left decrease size, right increases. Now take the Pucker Tool, and click on the outside edge of your image. See how it 'puckers up', or sort of shrinks? Takes a lot of patience, and experimentation, to pull this off, without it looking unreal. Check out all the other tools also. Down towards the bottom is the Reconstruct Tool. Using this tool undoes whatever changes you made. It's handy to undo just small changes, or you can undo the whole thing! If you like what you see, or if you want to save what you've done, to come back later, and make some more changes, go up to the right, and click on OK. If it's just horrible, or a experiment, and you don't want it, go up to Cancel. There are many other wyas to do this in Photoshop, which may be a good deal, as I can't remember if Photoshop 8,(CS), 9,(CS2), and 10,(CS3) were going to keep the Liquify Tool. I'm runnin' PS7.
Color of your eyes:
To the left of your workspace is the Tool Menu,(We've already established how to find the names of the tools. This works in all versions of Photoshop). At the bottom of the Tool Menu you will see two squares, attached to each other by their corners. These are the Foreground Color Box, and the background Color Box. The Foreground is the one to the left, and looks like it's on top. You should see the 'colors' White in one, and Black in the other. Doesn't matter which one, this is just a reference. Click on the Foreground Color Box. You will be presented with a Color Picker Chart. (Sounds pretty technical for a program that cost's $300 to $600 bucks, huh? lol!) You will see a large 'box' on the left, and a slider 'box' on the right, with all the colors of the rainbow, in various hue's. Let's just concern ourselves with this for now. Take your mouse cursor, and click once out in the middle of the large box. See the small circle, that your mouse cursor moves? Okay, clicking on any spot in the large box, for any color, or hue of that color, will put it in the Foreground Color Box, when we're done. Now back to the slider. You can click n' drag on one of the two arrows,(or triangles), and move it up, or down. See how this changes the color, and hue in the large box? This is how you get the color/hue you want. You move the slider, and click out in the box. You will see the result of your choice in the top box to the right, of the slider. You haven't made a choice yet, you're 'just looking'. When you think you have the shade that's close to what you want you eyes to be, go up to the right in the Color Picker chart, and click on OK. The Color Picker chart will close, and if you look at your Foreground Color box, you will see the color you chose. Now go to the Tool Menu, and choose the Brush Tool,(not the History Brush). The Brush Tool has two different brush styles,(Not quite the proper wording. The Brush Tool has many styles. I'll just go with this for now), the Hard Brush, and the Soft Brush. If you go up to the top header, and click on the Down Arrow next to the Brush size number,(example: Brush 35 >, but the arrow faces down), you will get a Drop Down menu with a slider on top, and different pre-determined brush size's below. Check the chart out, with it's slider to the right of it. If you run the chart down a bit, you'll see the brush sizes increase, then go to a small brush size, and increase again. This is because it lists the Hard Brushes first, then the Soft Brushes. I only use the chart to click on a Soft Brush,(any size), or Hard Brush, then go up to the top slider, and I click n' drag to the size of brush that I want. generally Soft Brushes are used for human skin, and eyes. Yes, it's experiment time again! Say you want to change your eye's to Green. Any shade, it's your choice,(Color Picker chart!). You have the color in your Foreground Color box. I would chose a Soft Brush, just large enough to encircle one eye. Then I would go up to the top header, and click on the side arrow next to Opacity. I would click n' drag the Opacity slider to about 20%. The I would click on the eye, and see what the change was. Not enough? Click again,(and again, and so on). First click=20%, second click=40%, and so on. You should do subtle changes. Remember what you did for this one eye, or write it down. You'll want to duplicate this for the other eye!
Blemishes, pimples, etc. Use the Clone Stamp Tool, or the Healing Brush Tool. The Clone Stamp Tool copy's from an area you chose. It's best to copy from an area, real close to the area you're changing. Go to far away, and the skin tones, shading, and texture won't match. To use the Clone Stamp Tool, choose the area to copy from. Now take the mouse and move the tool to it. Hold down the ALT key, and click once. Let go of the mouse left-button FIRST, before you take your finger off of the ALT key! Now move the tool to the area you want to change, and click. Two more things, before you go on with this tool. In the top header you will see Brush, Mode, Opacity, Flow, and Aligned. You can change the Brush Size just like you did for the Brush Tool. I would leave the Mode to Normal. Experiment with Opacity, leave Flow alone. The Aligned function, means that the spot you chose to copy from, will move with the tool. You move the tool a little bit, the spot the Clone Stamp Tool get's it's 'copy' information from moves with it. You can go up to Aligned, and click the box next to it, to turn this feature off. (Removes the check mark, so you can see it's disabled). Want it back? Click the box.
The Healing Brush Tool is relatively the same thing as the Clone Stamp Tool, but it's like the Clone Stamp on steroids!
The Healing Brush Tool derives it's 'information' from a larger radius of pixels around it, then uses a Algorythm to match up the best possible choice, to match the area it's being used on. You have a pimple? Hold the ALT key down, and click once on a area next to the pimple. Let go of the mouse key first, then the ALT key, Now put the tool on top of the pimple, and click. Viola! Bye-bye pimple!
Now for the Tutorials on Photoshop. These will expand the tiny bit of info I have given you, greatly. There are shortcut keys to use on your keyboard, to go past the clumsy way I have described. I just felt this would get you 'off the ground', and you would see what the 'longway' is, compared to the shortcut key way. (I believe it gives you more knowledge)
Tutorials:
1.http://graphicssoft.about.com/od/photoshoptutorials/Essential_Tutorials_for_Photoshop_ImageReady.htm
2.http://www.youtube.com/results?search_query=photoshop+tutorials&search_type=
3.http://www.karbosguide.com/ Go down to Photoshop Tutorials-Online......
4.http://www.photoshopessentials.com/photoshop-text/text-effects/gold/
5.http://www.perlentraum.de/undina1/toolbar.htm Basics for the tools in Photoshop 7, but can be crossed over to the newer/later versions also. It's a basic guide to tools in Tool Menu.
6.http://www.photoshopdigest.com/basics/creating-custom-brushes/
7.http://www.good-tutorials.com/tutorials/photoshop/?p=1 1545 pages, of 4 to 7 tutorials on each page!
8.http://www.psworkshop.net/psworkshop/forum/default.asp
9.http://www.atncentral.com/tutorials.htm