Hey I'm launching a brand next year. I have been preparing for almost two years because I registered everything, made the full corporate image, but I don't think you need that much. The first step would be to know your budget. How much money you have. Pre-order is great but they are covering just a part of the expenses. If you are paying for a website, renting a space in a store, buying ads then that comes to from your pocket until you see any profit.
I refuse to take pre-orders because I'm making brand new products and I can´t estimate how long would it take to produce, I'm so glad I didn't take anyone's money because so many things went wrong with the manufacturer!!! So, maybe just take pre-orders when you are sure that you will have the quality you ask for and have a trustworthy predictable cycle of production.
Also, I think you can sell in a physical store, not everything has to be sales online. There are many indie stores, galleries, etc. Meeting people in person is so great to convince them to buy.
About what to sell, If you are just starting to build a name, people are not likely to buy because they are loyal to your art, they are more likley to buy because the stuff is cute. So, think first what kind of design will appeal to more people (while it shows your essence of course). Have you seen how brand new singers start with a cheesy Christmas albums? LOL.
I studied Graphic design and our first step is to always know who are you selling to and what need does that satisfy.
Is your art for adults? Kids? Teens? Remember that kids require parents approval to pay you and teens don't have much money, they are likley to buy only cheap stuff. At least in my country going to a workshop to print your stuff is way cheaper than online suppliers. Printing workshops often suggest you what products thye have available: T-shirts, bags, pens, caps. And they are able to show you examples of their work so that you see the quality.
When you are making your ads be very clear about terms and conditions. Will you ship worldwide? registered mail? How long it takes. Refunds?