latitude longitude - Income data on a block level: how does a newbie do it?

I am a complete newbie at GIS data, and am trying to do the following:

1. Figure out census blocks in Miami-Dade (and several other counties) that have average household incomes > $75,000


2. Get the latitude-longitude boundaries of those blocks



3. Plot the blocks on a map

1 and 2 are the most important. What is the best way to get this data, and then the lat-long boundaries? Quickest, easiest, cheapest are of course the top criteria.