2 Options I can think of
Using just the 20 G to start up, I would think the only thing you can do would be one of the portable stand type of franchises, such as the "Dominick's" hoagie carts they have outside the Home Depot's or Lowe's (don't know if there is an exclusive contract for either). Another small one would be a portable Starbucks' outside a busy business complex.
Option 2 would be that if your credit is good you could use the 20 G as a down payment on a loan to get a small one in a building / leased space.
You definitley need to do research first. Besides start up costs, you'll need various licenses, health inspections, and if you plan on hiring help, you'll need to deal with the tax / workers comp facets as well as insurance. I would'nt do it in the middle of bumble f%#k FL either. I would find a growing area on the outskirts of a large city such as Tampa where there are possibilities of both smaller competition and a larger pool of patrons.
Finally, after research is done and you choose a franchise, contact them and see if they have any start up programs where they will provide initial start up assistance.