first, find a place you like. If it is on a sidewalk or street, you will most likely need a permit. Call or go to the City or Town offices that has jurisdiction - if not city or town, then County... They will tell you whether you need a permit.

For office buildings, look at the directory - usually on the first floor, near the elevator. Locate the "Building Management". You want to speak with the property manager. If the space is already in use by a company, you will most likely not be able to use it - but it is worth speaking with the Office Manager. If the space is not in use, it is likely to not have any furniture... so either keep looking - or rent some office furniture if you want it to look like an office.

You may also need insurance. What happens if someone gets hurt during filming or you accidentally break something really expensive? If you think it won't happen, I suggest you at least plan on it happening - unless you have lots of $ in the bank. Check with your homeowners insurance to see if they cover anything for you - or if you need a "rider" for each individual project.