The Florida Turnpike ends at Florida City/ Homestead area {Keys Gate}. This last exit (check Wal-Mart (includes a FULL food store) here, as well as a Murphy Oil gas station (cheap), and a very nice Golden Skillet
http://www.goldencorral.net/ ) will put you on US Route One South. A large sign at the end of the Turnpike says: "Key West 130 Miles"
US Route One starts (or ends) in Key West or Maine. In this area, it is not a super highway, but sometimes a one lane S and one lane N. Places to visit or indicated by mile markers.
The turnpike is the best way to get to Keys Gate/Florida City. Interstate 95 ends in the middle of downtown Miami, (also leaves you off on Rt One, but it is a long way from that point to Keys Gate.
If FFL....after you get rental from airport, exit onto US Route One (yes, the same) and go N toward Ft Lauderdale.....(There is a new ramp almost completed where you may avoid Rt one). In any case, your objective is to get onto Interstate 595 WEST toward the Everglades-which borders the airport on the N side.
You will pass exits for I 95 N & S, and right then watch for exits to Florida Turnpike. (It comes up very fast)
Follow signagge to Florida Turnpike South, and then watch for HOMESTEAD EXTENSION....which you will get on. The Fl T. heads S toward Florida City. Stay on till the last exit. (Another branch of the Turnpike ends at THE STADIUM {Joe Robbie??}...and Caldor Track....this will put you in N Miami....avoid)
If Miami, after obtaining rental, there is a toll road that will take you to the Fl Turnpike S. (Don't go towards I 95...opposite direction). Just follow signage.
If you can meander while driving down, there are a lot of interesting sights on Rt one along the keys. But the driver needs to attend to the road much of the time.