BEGINNER240SX

Okay guys, I finally got the money together to purchase my first 1990 Nissan 240 sx, I'm doing some research as to what my first mod should be. I'm all about performance but I don't know where to start, so I'm asking for advice, what should be my first modification under the hood, think cheap? I don't know much since I'm new to the Nissan family so talk to me like I don't know what anything means.