A ranked tier system where matchmaking would match people of a similar ELO/Rank? And allow them to climb by improving?
You are delving into the most complicated scripting techniques i know of. Im going to link you to Roblox Tutorials for what you need to read on matchmaking, leaderboards, and DataStorage
http://wiki.roblox.com/index.php?title=Leaderboards
http://wiki.roblox.com/index.php?title=Saving_Player_Data
http://wiki.roblox.com/index.php?title=Matchmaking