Sounds like you already have a business as a self-employed independent contractor, there really isn't anything you need to set up to make it an "official business." In your case there isn't a real reason to set up an LLC or corporation, which would protect you personally from the misdeeds of your business partners. And no reason to publish a fictitious business name statement.
If you are getting 1099's then the IRS is as well, and they expect you to follow the tax laws and declare all of your income. This will be subject to self-employment tax (employee and employer social security and medicare). Tax time is about to get more complicated for you.
You should immediately start tracking all your mileage, no reason not to claim all the business deductions you are entitled to.
If you are not already maxing out a 401k at your main job, then a Solo 401k would allow you to contribute more then a SEP IRA. Otherwise, a SEP is super simple to set up, and you can contribute up to 20% of your self employment earnings (after expenses and 1/2 of self employment tax is deducted out.
The first year we set up a SEP for my wife, we contributed $1600 and our income tax bill went down $800, so effectively a $800 contribution with a 100% match from Uncle Sam.
