There are plenty of sources of information to help you out. Training courses like this one, Excel Help topics, and the Insert Function dialog box can all help you learn to write better formulas.
The Insert Function dialog box
As you can see in the image on the left, you can choose what type of function you're looking for and then choose a specific function from a list. You can also get a description of that function as well as request Help on it.
Note If you can't find the function you're looking for in a particular category, try a related category. For example, note that AVERAGE, the function for arithmetic mean, is listed under the Statistical category, not in Math & Trig, but that RAND, the random number function, is listed under Math & Trig.
You'll get to try using the Insert Function dialog box in the practice session.