It's pretty hard to get lost as this is a small island with a few well-marked main roads. Okay, so we did go around in circles a few times, but we weren't paying attention!
The main roads across are Periwinkle Way, which you come onto as you leave the causeway (really long bridge). You'll run into Tarpon (not Tampon) Bay Road. Go right to get onto Sanibel-Captiva Rd. going to Captiva of course, and the Wildlife Refuge. Turn left on Tarpon to get onto West Gulf Drive which follows the water.
My parents tell me that in winter, driving on this island is sheer hell and damnation, as the traffic is horrific. They often park on the other side of the causeway and take a shuttle over.