You need to discuss this with your client. Gather some catalogue images of various styles of product photography and together come to some idea of what is needed.
Will the shirts be modeled? Will they be on a mannequin? Are they to be hung in a display? Do they have unique features needing emphasis? ... etc
Lots of unanswered questions and each one need a different lighting approach.
However, avoid flat one light setups with no modeling, avoid glare, keep the product visually separate from the background, don't crop too tight, don't crop too loose, etc.
If you don't have a clear idea of your proposed set then look at a the examples you collected earlier, pick one and try to replicate it. If you are not clear how then is when you come back to the forum for specific advice and instructions.
Just my two cents worth.