I not sure on what you mean when you say you don't know where to start but I hope this help
Do your t-shirt design in illustrator, vectors print better. Use PMS colors to design for t-shirt. Depending on the printing company you use, see how many colors you're allowed to us. The more colors, the more expensive. Make sure what applications they accept. Photoshop is a good program to use. If you use type then convert it all to outline. Find what format they (printing company) want the design in that they want sent to them.
Learn which fabrics print better, your printing company should have this information, so learn about them as much as possible, so you can better prepare your design for print.
Here's a few links that can help get you started.
Printing companies
http://www.ponyprinting.com/ - I recommend <3 They do all over printing!!!!!
http://www.heartcoreclothing.com/http://www.americaniconmerch.com/http://www.mammothprintshop.comhttp://www.tshirtpros.com/Companies that do wholesale that you can your shirts from the print on.
http://www.hanes.comhttp://americanapparel.net/- I recommend <3 Most t-shirt companies use them
http://www.fruit.com/ - Fruit of the loom
http://gildan.com/Good site to check out
http://www.howtostartaclothingcompany.com/This is a cool site to host your t-shirt site!
http://bigcartel.com/Note: Most printing companies will print on any brand (shirts) you desire and they will get the shirts themselves. Check out the printing company first and see what brands they print with. If they offer it, use American Apparel, and I recommend it
There you go!! If you need more help PM me.