>>76447518 (OP)
Cotton or cotton blend gym shorts with a 5 inch inseam. No gigantic basketball shorts. Solid color. No logos or sayings.
100% cotton t-shirts - one color. May have a single sports team or college logo on it. Nothing with more than 3 words on it.
If you are lean - visible bicep vein AND delt/tricep seperation, tank tops are permissible. Again, solid colors with minimal logos and no sayings/slogans.
What sneakers to wear is largely personal preference. Running shoes are comfortable but gay. Basketball shoes are youthful, but try hard. I have found that old school tennis or basketball shoes (lows, not high tops) are a pretty good compromise. If you can get a pair of canvas adidas or puma tennis shoes, thats a good, age appropriate, not try hard, not "Dad" look.
Socks are a mystery to me.
Do NOT wear a fucking hat indoors. Ever. Never. Under any circumstances. I dont care that youre bald. Have some fuckign dignity.