Absolutly nothing! It is so much fun to watch the little ones play T-Ball. It is really non competative and it's just for fun. They are learning to listen to a coach, hand eye coordination, pay attention to their surroundings, etc...but in a fun way.
The ONLY thing WE had to buy was the glove, the league here in our town supplied the coachs all the other supplies like bases, white chalk for marking the fields, the T, the balls, batting helmets, bats, everything and they were responsible for their kits and had to turn them back in after the season.
We decided to buy a batting helmet. Wait until you have practiced and find out what the league regulations are. Ours didn't allow certain types and the league actually provided some for our use. I worry about head lice so we bought one at Wal-Mart.
We also bought a pink T-Ball bat and a couple of the other girls had the same one and a few of the boys brought thier own too, but again, the league supplied the coaches with bats for our use.
We bought shoes with cleats, obviously she didn't need them (Our league had a "no out" rule for T-Ball so everyone made it around the bases) but it made the outfit look more like a big kids uniform.
We bought the baseball pants the second or third season she played because everyone else had them, they were about $15 at Hibbets. The tee shirts were supplied by the league, part of the fees, and they were primary tones like navy, emerald green, red, dark purple, orange, bright yellow, etc...with white lettering one year and maybe black lettering the next year, so we have white ball pants and black ball pants.
I can tell you now that ours was the flower picker for the first winter and spring seasons but after that she was the gymnast, always turning flips in the field. The coaches were good at reidrecting all the kids back to the game. The kids get bored after standing in the same place waiting for a batter to finally hit the ball after 4-5 tries of having it pitched to them and then when they finally get it on the T and hit it the kids would go to work.