how old are you? if you are young your metabolism is generally gonna be fast and its gonna be harder to gain. as you get older it becomes easier..
also your job is a factor.. if you are digging ditches all day and sweating off everything and burning off fat and muscle you won't gain.
cardio, this is a biggie... if you want to gain you should reduce your cardio, you can always re-introduce it down the line
a supplement like creatine sounds like your best bet
and finally read the bulking thread in my signature below.. that will give you a good guide.
Thanks. I'll be 29 in a few weeks. I have a deskjob and try to conserve as many calories as I can when not training. I will read your guide.