When you want to insert a formula, click the Function button or select Insert > Function from the menu. This will open a dialog box that has all the functions sorted into categories. This will help you find the function that you need.
If you insert a function this way, the function wizard will automatically start up and guide you through the process of construction a formula.
You can also start the wizard at any time by click on the = sign on the formula bar.
If the wizard is not enough, look up the function in the Help file, which usually provides detailed information on it.
If all else fails, come to this message board and ask for help with a specific function.
Good luck,
-Ben