2002 Chevy Suburban..Wont start...Has full battery....No other problems?

My SUV wont start...After doing a BUNCH of researching. I know it is common and has something to do with the security system... When it doesn't start this is what I have noticed...1 it has been sitting for a couple days. 2 on a small hill....3 might be really hot or really cold...4 a sound coming from the small speaker by the door....like the on star speaker.... I dont know how to reset this.. people say they are doing a series of things with turning the key on and off...Help has anyone had this problem and found a fix for it????