Thats a tough business you're getting into, real tough. I got some experience with it and I would suggest you
1. Test your ideas first, see if you have a market.
2. Start Small - Only do 1 or 2 designs in a couple of colors
3. Stay basic- If you are doing t-shirts do 1 screen, with jeans no more than 5 pockets, keep your colors conservative for starters
4. Do business with local vendors. Don't get into overseas companies to start. You want people you can reach if you have problems.
5. Sell your stuff first. Sell it out of your home, trunk etc. Don't approach stores until you know people like your work. Stores will try to beat you for the lowest prices.
6. Don't bring in more people than you need. 1 partner sounds right.
7. Read everything you can about the business
8. Get college students for your designs & artwork if you can't do it yourself. They work cheap and they want the experience.
9. Get your stuff copyrighted.
10. Don't let a small time company talk you into licensing deal, keep your own stuff.
11. Lots more to it but last not least: READ THE FINE PRINT. DON'T SIGN ANYTHING YOU HAVEN'T READ.