You’re likely to see at least one of the Big 10 Rock-Forming Minerals in nearly every thin section you look at. Here they are, in alphabetical order for your convenience.
Ca-rich plagioclase feldspar (anorthite)
Na-rich plagioclase feldspar: (albite)
An honorable mention must go to the chlorite group minerals. While not one of the Big 10, the chlorite group minerals—often lumped under the catch-all name chlorite—are extremely common alteration products and thus turn up in all kinds of places. It is well to learn to identify chlorite early on; it’ll save you trouble down the line.