The University Of Miami has a really really REALLY great timeline of the history of Dade County and the city of Miami (and surrounding areas). Dade County was also included Martin, Palm Beach and Broward counties.
And from the date of 1819 when Spain ceded Florida to the United States, the timeline begins. And the earliest history of Miami and Dade County comes in the form of the Cape Florida Lighthouse that was completed in 1825. Its located at the southern tip of Key Biscayne and exists till this day (for the most part, continue reading). The lighthouse was a lookout point for ships crossing near the coral reefs. The lighthouse was unsuccessfully set on fire by Seminole Indians (who we were at war with) in July of 1936 on the same year that Dade County was created. Despite managing to survive, the lighthouse was in really bad shape. It was then rebuilt from its original bricks and relit as of 1847.
So checkout the entire timeline here. Its a great read to get you up to date on the main key points of our South Florida history. Again, not all of it, but some great key points.