Your wholesale price (what you sell to stores for) should be roughly double your cost (what it costs you to make a shirt).
So if your cost is $5, your wholesale is $10 and the store will sell the shirt for a minimum of $20. Stores usually mark up your product between 2 and 2.5.
That extra $5 for you between cost and wholesale covers your overhead, marketing expenses, etc. and hopefully give you a profit as well.
This is the same pricing model that everyone uses. If you are starting out I would recommend you also research your direct competition (shirts that will be similar to yours in style and quality, and appeal to the same customers) and see what their prices are. If your competitor is selling retail for $40, you can charge $15 for wholesale and still be competitive. You can (and should) do this even if your cost is still only $5.
Never charge less than double your cost for wholesale or you won't make enough money. It might be ok to "get by" with no profit for a while, but you'll need it later and it's going to be very difficult to raise prices later.. so have your pricing correct from the start.