i want to start a business selling t-shirts with my drawings on them but i'm not sure if i have to copyright them first.

I am not licensed to practice law in your state.

These comments are made for educational purposes only and do not constitute legal advice. No attorney-client relationship exists between us.

You need to get the copyrights before you start selling. It's fairly simple and not expensive. See the link.

The below does not constitute legal advice, does not form an attorney-client relationship, and should not be relied upon to take or refrain from taking any action.

You don't have to register your work before you use it in this fashion. However, it is advisable that you do register it, since pre-infringement registration is a prerequisite to statutory damages and attorneys fees against an infringer.

Disclaimer: This post does not constitute legal advice and does not establish an attorney-client relationship.

I agree with Mr. Nelson's comments with the following addition:

Registration is a prerequisite to your bringing suit for copyright infringement. While registration is inexpensive, bringing suit is not. Accordingly, you should incur the expense of registering your designs or drawings only if you believe there is a reasonable likelihood that you actually will bring suit if you discover any infringement. If you know that you are unwilling to incur the cost, time and disruption of bringing suit, then there is not a lot of point in registering the copyrights.