Apply tint according to the state legal limit. The majority of the U.S. states say 25-35% is the limit, but some states(like Florida) might have 50% as the limit.
Most tint is the stick on kind. To apply it yourself make sure the window is cleaned thoroughly and that you apply it on the INSIDE of the car. If you do it wrong, the film will bubble and peel off easily. If you think you'd mess it up, then have a professional do it for like $30-$40.
Yeah, definitely dont get the stick on kind, that looks horrible after 3-4 years. I got like the mid priced tint. The place I got it at also gave me a lifetime warrantee so you might want to try to find a place like that. I've gotten mine slightly darker than the legal limit (it's actually around 28%, FL has one of the lowest limits which makes sense given the scorching sun all year round). Although maybe I just look way too innocuous.
NOT the stick on kind. It peels off.
And not too dark. A friend of mine got dark tint and we got pulled over for it TWICE on a trip to Tampa. Make sure the people who do it know the legal limit of darkness in FL. They're strict.