If you're flying in and out of Tampa and WDW is just one of your stops in Florida, it makes sense to rent at Tampa Airport (not through
Alamo at the Dolphin).
Tampa Airport has all the major car rental companies. Click on "Rental Cars" at Tampa Airport's Ground Transportation webpage:
http://www.tampaairport.com/ground-transportation
You need to decide what size car you need. Normally, you reserve a car category (such as midsize or fullsize), not a specific make and model of car. Be aware the your idea of a car size and the rental car companies' categories may be different. For example, "midsize" cars can be quite small.
You'll want to look for a good rate. There is not any one company that's consistently less expensive. You'll find all sorts of rate discussions if you search this forum.
I strongly suggest never booking a "prepaid" car rental that cannot be changed or cancelled. If the airline changes your flights or you find a better rate, you would want the flexibility to make a change.
There are a bunch of other things to read up about, such as gasoline (car should have a full tank and you should return it with a full tank), insurance (most renters can safely decline the insurance that car rental companies try to sell because they're covered through their regular car insurance), and tolls (no tolls between Tampa and WDW, but plenty of toll roads in Florida).