If it will be your first rewards credit card, we would recommend applying for a no annual fee credit card with a fixed cash back rate on all purchases, without any changing categories and enrolments. Cash back rewards are easy to understand, manage and redeem. You will always know how much you will get back from this or that purchase.
When you get acquainted with rewards, you can consider other types of rewards programs: points, miles, programs with rotating shopping categories, programs with additional rewards in specific categories programs with flat rewards or programs with tiered rewards. It is important to know limits and caps of each program and be sure you will earn maximum on all your expenses.