You'll probably start with simple geometric series, which are easy to sum and determine convergence, and (if it's AP Calc) go up to Taylor Polynomials, which are used to approximate transcendental functions (among other things). Make sure that you learn all of the tests for convergence, and try to recognize patterns in the series. Often times you'll be given series that resemble one of your formulas, and you have to manipulate the series so that it resembles that formula, at which point you can apply a test.