Choose a specific country or period of interest. Renaissance architecture developed first in Florence and then spread throughout Italy. Then it cropped up in the Netherlands and spread from there to the rest of Europe. So if you're interested in early Renaissance architecture, focus on Italy; for late Renaissance, focus elsewhere in Europe.
Concentrate at first on the three most notable architects of the time. Filippo Brunelleschi is considered the first Renaissance architect. Leon Battista Alberti was an early designer in Florence and Mantua who wrote extensively on art and architecture. Andrea Palladio specialized in villas rather than churches and also wrote on architectural theory.
Use what you've learned about the three big names from Step 2 to understand how their architectural theories relate both to their classical predecessors and to their fellow Renaissance successors. Renaissance architecture was inspired by the temples and buildings of the ancient Romans.
Buy or borrow some books on the subject. "Renaissance Art and Architecture" by Gordon Campbell, and "Discoveries: Architecture of the Renaissance," by Bertrand Jestaz are good places to start.