Before you set up your website on the Internet you need to decide if you'll sell only to individuals, t-shirt merchants or both online, this will in part determine how you set up your website and what you set up on it to make money. You’ll also need to know where your possible targets are on the Internet, that is who need screen printing or embroidery, where are they etc.
You'll need to register a domain name for your business (with a domain registrar), you can get that at
Register Domain Names at Register.com | Domain Name Registration & Web Hosting Services,
Netfirms - Web Hosting, Domain Names and E-commerce for Small Business etc, you also need to buy a web hosting space (from a web host) where you'll put whatever content you want on your website, you may ask your domain registrar or your web host on how to link your domain name with your website.
You should also look at similar websites online offering the same services/products you offer, you want to get a feel for how you should set up your own website.
So you need a domain name and a web hosting service,to store the list of products you sell and take orders you'll need a store, a shopping cart, a payment gateway and some form of shipping service to deliver your products to your buyer, you may get these services from those mentioned early on by others, just do a search for any of those services and check them out to see if they meet your needs before you choose and pay for any one of them. If you are good with website design and coding you can set up your site by yourself or hire someone else to do it for you, you may get someone at
Elance - Outsourcing to freelance programmers, web & logo designers, copywriters, illustrators and consultants | Elance.
Once you've set up your website to take orders the next major task for you is to get to your targets wherever they are and give them exactly what they want, this is why it is so important for you to know who you'll be selling to whether they are individuals or merchants, both targets will buy t-shirts but for different reasons, you'll get to your targets when you use targeted ad media and promote with ads and/or content that sell.