My guess (not being familiar with MD statutes in particular), is that if it's anything like NY, you MUST apply for a CERTIFICATE OF AUTHORITY, either to collect SALES TAX, or SERVICE TAX, all for YOUR taxpaying purposes come year-end (April-15).
It's a start...
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Your question states that you are under 18. If you are a minor and you are not emancipated, you should talk to a lawyer about the ramifications of entering into contracts. Do you get work assignments from someone else? Or do you get the business for yourself? Depending on your answer, it may make sense for you to establish a small business entity. There are several issues here that a lawyer can help you with--many lawyers will give you a free consultation which will help you figure out where you stand and what your next step should be.