Freelance income can be put into a tax advantaged retirement account. You do not need to be incorporated as a business, though I would highly recommend forming an LLC especially if you're planning on turning the business into a full time career.
Forming an LLC is actually really simple. In Michigan it's a single page and $50 to form an LLC, and I would bet it's just as easy where you live. It's better to form one sooner rather than later, for liability purposes and for demonstrating a reliable income history if you're ever applying for a business loan, etc.
That being said, I prefer Roth IRAs over traditional IRAs.