I am going to take a guess that you are taking a physical inventory and want to just count full cases and then have Excel convert that into units for you.
Some tips:
1. All Excel formulas start with the equal sign "=". Start with an equal sign and then you can click on cells to build equations/calculations using the operators like +, -, *, / etc. Excel also has hundreds of automatic functions like SUM, MIN, COUNT, AVERAGE, etc. There is a little icon next to the formula bar (Fx) that you can press to bring up the function wizard. Probably not necessary for what you need.
2. Assuming I guessed your problem there are two ways to solve this issue. First lets agree on the structure of your Excel document:
Column A - This is where you are going to record the SKU number which I assume you will need
Column B - This is where you will record the number of full cases you found in location during physical inventory
Column C - This is where you will record any loose pieces found in location (if applicable). This is in case you come to a location that has 10 full boxes each with 12 units per box and then there are an additional 7 units not in a box resting on top.
Column D - This is where you will record the units per case (per SKU). If you have a report somewhere that lists the units/case for every SKU you will not need to record this information manually, you could easily use a "vlookup" function in excel to automatically return the units/case value for each SKU. For simplicity sake I will assume you don't have a report with this info so you have to manually record it.
Column E - This is where you will calculate total units found in each location. Assuming you used the same table organization I suggested above and that the first row was used to name the columns your formula should looke like this:
=(B2*D2)+C2
Once you enter that in you can highlight the cell and drag it down to auto-fill the entire report. As you drag the cell down it will automatically change the reference of the cells to match the current row