When you first start to use a new function it can take a while to get used to the syntax (layout) of the function. Excel assists you with the Insert Function button (the fx button between the name box and the formula bar, above the column letters).
You can click the fx button to select from a listing of functions or you can type = followed by the function name and the open bracket and click the fx button to get help on that specific function. For example, in the formula bar type =VLOOKUP( then click the fx button.
This feature can step you through creating the function, it explains what the various components are used for. As you create the function in the Insert Function screen the changes appear in the formula bar.
Note: some syntax components of functions are optional. Required component's names are bold in the Insert Function screen. The Help system also explains the components and whether they are optional.