If you have decided that it's time that your business becomes incorporated, and that a Limited Liability Company (usually referred to as an LLC) is the best entity, then your question is going to be: who do I use.
First, you should know that forming an LLC is not rocket science. Any halfway decent attorney or online incorporation company will be able to handle the paperwork.
What it really comes down to is cost and the amount of hand-holding you need through the process.
Your three options are (in order of descending cost and ascending effort):
1. Have an attorney handle everything.
2. Hire an online incorporation company.
3. Fill out the paperwork yourself.
Hiring an attorney: This option costs the most but requires the least effort on your part. You tell the attorney that you need your business incorporated or need an LLC, and you'll soon have one. Any attorney that is fit to hold a license should be able to do this with no problem. If there is a problem, then you can always sue them for malpractice.
Hire an online incorporation company: Hiring an online incorporation company is the most common path. It costs less than hiring an attorney and your company will be just as legal. You'll have to learn a bit more, like what is a registered agent and do you need to pay $125+ per year for it. All incorporation companies perform the same basic service of complying with state law to form a business entity. The major differences are price and whether you can order services individually or must take a more expensive package.
Do it yourself: It is possible to form an LLC yourself by filling out what are called Articles of Organization. A legal LLC in any State requires the filing of the Articles and payment of a registration fee to the State. The forms are actually relatively simple to fill out--most are only 1-2 pages long. Here's a list of where you can find Articles of Organization by State.
In addition to filing the articles, you will also pay a fee to the state in which you're forming your LLC. These fees are unavoidable if you want to incorporate. Fees are set by each state individually, and range from under $100 to several hundred. Some states charge extra for expedited filing. When companies advertise "Incorporate for only $49!", look to the fine print--because the $49 is the company's fee only. The state gets a fee in addition.
It is possible to save hundreds of dollars by forming an LLC yourself. If you're willing to spend a few minutes filling out a 1-2 page form, you can form your own legal LLC without a lawyer.