First off you need to create a name. Think of something catchy not boring.
Then you need to create a logo once again not something boring! You want people to see this logo and know what you do if you are designing there t shirts they are not going to want a logo from you that looks like a 3rd grader made it.
You biggest expense is going to be getting licensed and deciding if you want to open an LLC of what route you want to take.
You will need to brand yourself facebook, your own website, get your name out!
Choice between the two.
1. You will have to find a supplier that is going to make these for you if you are selling them. You need someone that is going to give you a good price.
2. You can buy all the equipment your self. This could be really expensive there is a lot in this that people do not see.
Then you need to sell, sell, and sell some more!!
There is a huge profit margin in this, every business needs shirts, signs, or advertising materials.
This is just a simple break dow it could take a long time to establish yourself.