Well first off, welcome to TSF.
Second, congratulations on taking a leap into the business.
Lastly, to answer your question, what kind of shirts do you want to do. Do you want to mass produce shirts? Do you want to do small runs or one offs? What did you consider when you thought of this business (screen printing, vinyl, transfer, dtg). Once those questions are answered we can move forward lol.
To start, I personally chose screen printing but to each his/her own.