A lot of it may depend upon just what type of work you do and what role you take. What type and scope of work do you plan to provide? Are you primarily an independent installer providing subcontracted labor to existing contractors? Or are you looking to do turnkey installs yourself? Will you sell equipment or hardware or limit it to labor? Is it just you or will you start with or add employees? Will you have a physical business? Maintain any inventory?
The licensing and legal requirements vary from state to state and in some cases from one county or city to another. Some states have low voltage contractor licensing, but others don't and even in states that do have it, it is not always mandatory. In some areas you have to be very cognizant of unions, you may not be in the IBEW but that doesn't mean they won't have significant control of or role in your work. Working here in the Atlanta area can be much different than working in New York City or St. Louis or Chicago in that regards, even in aspects such as who pulls and terminates cabling. Then there's the usual business license, Federal EIN, resale number, general commercial liability insurance, workers comp insurance (if applicable) and so forth. The company can take about any legal form, the taxation and potential liability may factor in that decision.
There's also the physical side as in vehicles, tools, supplies, test equipment, etc. Of course, how extensive this inventory is depends greatly upon what scope of work you provide and what role you want to serve. Back in the office you probably need some form of keeping records, preparing invoices, tracking receivables and payables, etc. In addition, you typically need to have some contract forms developed, perhaps some marketing materials, maybe letterhead and so on.
Having a good business attorney, accountant and insurance agent were of tremendous value in helping me get started and keeping me out of trouble. However, in many ways setting up a business is still probably much easier than getting the work and developing relationships with clients and suppliers.