There are plenty of step-by-step guides for new-business-startups. Your state may also have a brochure with a check-list of state and local requirements.
By setting up an office in your home, you may trigger insurmountable zoning requirements. Although as long as customers never go there and you have no signs or externally visible evidence of a business, the authorities may let it ride.
Having a separate room set aside for work does help. You can "go to work" and more importantly "leave work." I started on a shoestring, (laptop and cell phone) but quickly found that I needed a separate phone line (with an 800 number) and a separate computer for business use. You do not want customer data on the same machine that you use to look at internet porn. Of course, if you keep any sort of customer's confidential information, it requires a whole extra level of physical and electronic security.
Although it seems convenient Do Not EVER give out your cell phone number to customers. Have a land-line for the office and if you need to roam, forward calls to your cell. Otherwise they will NEVER leave you alone. I stopped giving out my cell number five years ago, and it's still floating around out there in cyber space. I even had a dickhead customer call me during christmas dinner this year.
As much as possible, NEVER keep paper records. Get a fast scanner and a shredder. Scan everything and file it electronically. (Multiple automatic backups, of course.) Your accountant and lawyer will tell you what paper you actually need to keep. You won't BELIEVE how fast paper files will fill up your home.