Some companies require me to bill through a business or agent, this is my original reason. I'm a little afraid I'm going to get in over my head.
There are many companies that offer these services as a do it for you. I would recommend you look at a local business vs the services that are offered on line. I am in the LA area and just set up a corporation using the local resource. If they are legit, they will all charge within the same price range. I would look into the LLC vs a CCorp. If you wish to do it yourself, the forms and procedures can be found on the State of California's website. Nothing too complicated, but the process is time consuming.
You can use a convenient online legal service site such as legal zoom. These online legal services will be able to help you register in any of the 50 states, and you will also be able to file taxes and annual reports through them. The services fees will vary by state.
If you are considering converting your LLC to a C-corporation or S-corporation, it is recommended that you consider Delaware. Else, if you just want an LLC, your home state is fine. You can also consider Texas, Wyoming, Nevada, Washington, and South Dakota, which have no state corporate income taxes.