As this post comes from a new member, I hope that we're not going to see another new member posting soon, with the 'perfect solution' to the questioner's needs. (i.e. the old two-part spam trick).
However, assuming for the moment that this is a genuine post, then there's not really a great deal more needed other than:
1. Suitable premises to conduct the business from (or, say, planning consent to operate the business from home) ;
2. An appropriate power supply to operate the printing machine(s) and any other equipment needed ;
3. Limited company status if there would otherwise be a major risk to the business owner's finances if anything went wrong ;
4. Possibly third-party business insurance to cover risks such as a customer suing on the grounds that a T-shirt, or the inks used on it, has resulted in a skin problem ;
5. A contract with a courier company to be able to despatch items at the lowest costs ;
6. Depending upon the scale of the operation, possibly some packaging machinery (although, initially, just a supply of cardboard boxes, etc might be all that's required) ;
7. A banking arrangement to accept payment by debit/credit cards ;
8. A stock of T-shirts, in a decent range of colours and covering all sizes from a small child's one up to an XXXL ;
9. Loads of stock designs which customers can choose from and modify ;
10. Software which will allow customers to design their own products online ;
11. A website, with an online sales facility, to sell the T-shirts through; and/or
12. An account with an online sales platform (e.g. Etsy or Folksy) to sell your products through.