bind "F1" "m4a1" - buys a colt m4a1 ... then when you're in-game, press the F1 key at the beginning of a round and it will buy you a colt m4a1 if you have the money. That goes in your "config.cfg" file
If you open up your "autobuy.txt" file in your ".../counter-strike/cstrike/" directory, it'll tell you the syntax for everything you can buy. Hope that helps.