What is wrong with my 1994 firebird?

i bought a 1994 pontiac firebird at an auto auction a week ago and now it wont start. first the second day i had it i drove some where and shut it off and it wouldnt start again so i push started it and it ran fine. the radio and everything else electrical worked, but just today everything died as far as electrical. the radio wont turn on, and the power windows have no power, etc. at first i thought it was just the starter because it wouldnt start, but now im thinking the battery is going bad and didnt have enough juice in it to crank the starter because the solenoid does click when i turn the key. someone please help me it is my first car and i dont know much about it.