I have done this. If you are asking the questions that you are asking, then you haven't done enough research. you need to answer these questions first:
1. what is your market? your target market will dictate what blanks you will want to print/embroider on. For ex, if you have a hiphop style market, you don't want a tight fitting hipster look....you will want bigger sizing.
2. You need to order a bunch of blanks to sample. When I started, I sourced hundreds of garments and got opinions on them from people I trusted. Hats, hoodies, short sleeved, tanks, etc.
3. designs aren't too difficult to get into the right format. There are thousands of good artists out there that can create your ideas in a vector format...which is what you need for screen printing. For embroidery, you need a person that can digitize a design for you.
4. Spending the money to copyright is largely a waste of money, unless you really have something catchy that is very unique to you. I would trademark my company name, logo, and catch phrase, but the designs are tricky. most of your designs will be lucky to earn as much as it would cost you to copyright it. Fashion is a tough business. There is a ton of copying going on.
5. You may find a good designer on fiverr, but you get what you pay for.