You do not say if you have been to Orlando before or not, but if you have not think very carefully about WHEN you go, but also WHERE you stay.
The theme parks are spread out quite widely and getting to the theme parks can be difficult if you are staying in a remote rented house, or a hotel some distance from the various parks.
For example the Disney complex is HUGE, about the size of Manchester, with the various parks (and hotels) spread all over the site. There are major roads linking them all together.
Without a car it can be difficult and time consuming getting from the place where you are staying to one of the theme parks so as I said think carefully where you stay and how you plan to get about.
Some hotels do offer shuttle buses to the parks, but that can mean a lot of queueing and waiting around, so having you own car can make the holiday so much better.
The area called International Drive tends to be centre of the theme parks area, with lots of hotels and places to eat, but even these are some distance from the Disney parks.
The Universal parks are nearer to International Drive, but even they are not walking distance, though a bus does run up and down international drive.
There are loads of Orlando "advice" sites on the internet if you want answers to specific questions.