Might repeat a few points, so bear with me:
1) Local jurisdictions issue business licenses. Normally, you don't need one to do a random garage sale, to sell the 4 extra puppies your dog produces, or to sell the 5 scarves you knitted last winter. But once you are trying to regularly and routinely make money outside of a normal job by making something or offering service for something (employee, contractor excluded), you normally need a business license. Each area will define it so there is no precise way to tell you.
2) Once you have a business license, you may also need a state tax ID and a federal tax ID. The state department of commerce can tell you what the requirements are. You might need a federal one if you are selling outside of the state due to interstate trade.
3) You don't want to make sales and not report them as income since that's tax fraud. As long as it's really a hobby (and not considered a for-profit activity or your primary source of income), they try to ignore it and let you report it directly on your 1040. But once it's obviously a business, there are new issues. Depending on how you handle your activities (hobby, sole proprietorship, partnership, corporation) you may have to file taxes in a specific way. SEE
http://www.irs.gov/uac/Business-or-Hobby?-Answer-Has-Implications-for-Deductions
4) Most of this information can be gotten free by calling the IRS, or getting pamphlets or instructions from the IRS, commerce departments of the state, etc. Whether you need a lawyer or tax accountant is up to you.
5) You want to keep very detailed records. Generally, business income can be offset by business expenses. Fish sales can be offset by fish food costs, the electricity to heat the tanks can be deducted, the cost to advertise (have a website), to have a business card, etc. Even a portion of the house can be offset. Bad records means the taxable income might not get much offset and you might owe taxes when you shouldn't. Good records might let you offset a lot and only pay a small amount of tax. In any case, start reading up on that so you aren't caught in a jam later.