To be exact, it is best to go to the gym first, and then play basketball. Playing basketball is aerobic exercise, and fitness is weight-bearing exercise. The wisest way is to do strength exercises after a short warm-up and then do aerobic activities. If you put aerobic exercise in the first place, because it can reduce glycogen reserves and devour your strength, then your weight may not decrease, but will increase. On the contrary, if you do strength exercises first, you will soon reach the required state and get ready for aerobic exercise.