I don't know a lot about the t-shirt business yet but I do know a bit about business.
You already have a company name and logo ready to go, and I am sure you're excited and want to start printing up shirts and selling them.
If you know for a fact you will take this idea past hobby and want it to be a revenue generating asset for yourself, you will definitely need to protect your right to be the sole owner of your brand and it's designs. Getting your logo trademarked will make sure that some other company cant knock off your stuff without getting sued by you. It doesn't prevent them from trying, it just gives you grounds to sue and win.
You don't need a business permit to sell your shirts. You can create an online store, or website and sell merchandise that you've created all day long. Just make sure if you do it as an individual that you keep good track of your sales come tax time.
To keep this from getting very long I would suggest this.
Look at your end goal and work backwards, by creating a process list.
For example:
You want to have a T-Shirt store online, you have a logo and you have a company name.
You will need to copyright your company name (to be legit and win any infringement case if your stuff takes off) and you'll need to trademark your logo (to make sure competitors or 3rd party knock offs think twice about trying to use your company in any way without involving you).
You will need to decide how you want to sell your shirts. Do you want them to buy directly from a print distributor like cafe-press or do you plan on preprinting a mass quantity to fulfill orders?
I suggest you look at how much you currently have to invest into this idea, and make choices that make sense right now. As your income increases or you see return, you can expand your operation to suite your needs.
Let me know how things go! I too am looking to start a brand.
-Larry
