L.S.
Hi - another Anoka mom here! I started a business this year and decided to go with an LLC. It cost $160 to register. If you are a sole member LLC, you can file your taxes using your SSN so you don't have to apply for a separate TIN. If you plan to have employees, or business partners, it starts to get more complicated and I would recommend consulting an attorney or accountant at the least.
You can also do your business as a sole proprietor, in which case you'd have to register your business name (DBA) with the state, which last I checked was $25. But then once you get the paperwork back, you have to file it with a newspaper in the legal section 2 issues in a row. When all is said and done, you'll be spending nearly the $160 anyway. An LLC gives you a little more legal protection than a DBA too.
If you plan to sell taxable goods, you will also need to apply for a sales tax ID number through taxes.state.mn.us. There is no cost for this.
A great resource site is SCORE (score.org) where you can even hook up with your own personal business mentor.
I also believe that if you are doing your business using your full name, for example, John G. Smith Painting, you don't event have to file a DBA.