There's several things to note about your question. First all you need to create a website is a pc, but to get others to see it you'll need to host it, either on your own server or (better) through a webhost.
The reason i say a webhost is better is that for a beginner to use his own pc will require a lot of learning and a pc that is always on.
Next, it depends what kind of web site you wish to build, a simple personal site could be made with a free web design programme, or by using templates.
Here is what i suggest as you are obviously an absoloute beginner:
Start with a free hosting account from byet internet, the reason is that it is as near to proffesional as you can get for free and is full featured, so will provide you many ways of creating a site or sites while learning. http://byethost.com/index.php/free-hosting
Now, the username you choose upon sign up will also be your domain name, so for example you choose the username 'robert' your first domain is www.robert.0lx.net
You will also be sent details of your control panel log in, which will use a different username that they will mail you, plus the password you chose at sign up.
First, check your site is visible by going to your domain name, then log into your control panel.
Inside, you will see several different links for different options.
(This is typical of all web hosting panels)
You can choose to create a new domain name if you own one, you can either buy a .com for example or use a free one from http://www.co.cc.
Once you have created a domain name with any provider, you will need to point it to your hosting.
In the control panel, select add on domains, then enter the name of the domain you have created. It will tell you which nameservers to use, something like ns1.0lx.net and ns2.0lx.net
Then go to the site you created the domain with and log in, set the nameservers, then wait sometimes upto 48 hours for worldwide propogation.
Whilst waiting, you can get used to using your hosting by using the site you already have.
In control panel there are many features. The filemanager is for uploading your own sites if you have any, to get used to it, click filemanager, click htdocs, you will see index.html, delete it, then go to your homepage in a new browser tab. You'll notice the homepage has gone!
Now go back to your control panel and you will see a link for a free web builder, click and follow instructions!
Once you've played with that one, you could create a subdomain, then again in filemanager under the web builder link, you will see the fantastico installer link. Click that, view some of the sites available, choose one then follow instructions to install you own forum or blog or shop etc!
On one hosting account from byet you could host upto 6 add on and 6 sub domains, plus you could have a web site linking to a forum both on just one site.
Once you've got accustomed to byets hosting, you should be ready to upgrade to paid hosting and byet start at around 3 dollars per month (About 1.50 pounds).
Note though that byets free hsoting is supplemented by placing ads on your control panel log in, and sometimes on your site, and an easy way around this is to sign up for one of their free re-seller accounts, which gives you the same hosting but also gives you a separate website to give away free hosting to others, and you put your ads on. Then, you sign up to one of your own sites and take the ads off! Have as many sites as you like for nothing, but get used to one site first!