You need to go into Roblox Studio in order to make games and avatar items.
If you go to the create section on the roblox website, you just have to download Roblox Studio unless you already have it and they'll let you make games and t-shirts for free.

For the t-shirts as well, if you want to charge money for them, say 2 robux per t-shirt, or really just distribute them on the marketplace, then you have to pay Roblox 10 robux.
If the t-shirt is just for yourself and nobody else can wear it, then it's free to publish

Also, we're reviewers, so we don't have the power to unban you. Try contacting Roblox.