Here in CT, one need only register with the Secretary of State (and pay the filing fee) and create an operating agreement (which doesn't get filed with the SoS). There are plenty of operating agreements on line that you can use as a template. Basically, the operating agreement spells out things like ownership percentages and voting power, how decisions will be made, how the profits will be split, when you'll have owners' meetings and what will happen if someone wants out (buy/sell provisions). Obviously, this is all much simpler if it is a single member LLC.
The rules in your state may or may not be the same. You should conduct your own investigation.
Living an analog life in the Digital Age.