Welcome to Horse Forum! :)
Do you know anyone around you that rides? That would be the best source for identifying a lesson barn near you or at least a starting point. I'm going to assume you don't which would explain why you were looking online. I wouldn't worry too much about the negative comments, you'll find a lot of people enjoy spreading malicious rumours about competing barns. It's very childish but unfortunately all too common ... at least where I live.
Are you looking to ride Western or English?
Reach out to the barns you had in mind and see if their instructors are certified. Narrow the search down further if any of them have tons of experience working with adults, or even have current adult students. Ask to visit the barn to get a tour. Does the facility look organized or is there tack (equipment) all over the aisles and everything just looks like a mess? Ask to approach some of the horses, do they seem friendly? Make sure they have horses that are a good physical match! Maybe see about sitting in on a lesson, just to see what the instructor is like.
Pricing is going to depend on group, semi-private and private lessons. I have no idea what typical beginner lessons cost down in Miami, just shoot a quick email to a handful of barns to get an idea. See if there are any horse forums in your area, or pony clubs - they are usually an excellent resource for finding instructors but again keep in mind, A LOT of people have agendas to push. In most cases you're just going to have to get out there and try some barns... :)