Well I would not have believed it if someone had told me but I just did a number of searches and I could not find any free pattern drafting instructions for a t-shirt which strikes me as crazy as the pattern is so simple to make. Loads of sites for making bodices, trousers and skirts but nothing for t-shirts. Insane.
Anyway, my suggestion is a quick visit to your local library. There are a number of really good books out there about pattern drafting. My personal fave author is Anne Haggar (not sure if I have the spelling right), but Helen Armstrong Jones is very good and Winifred Aldrich practically wrote the pattern drafting bible. Connie someonorother who's last name eludes me for the moment has very simple to follow instructions in her book. Best bet.. go to your local library and ask the librarian what pattern drafting books they have. Failing that, buy a commercial pattern.