my advise (much has probably been said...):
start by buying bargain books to see what characters you like, then read up on the newer stuff and support your local brick n' mortar (I understand you can get the same book on ebay for $1, but...) to see if you are going to want to begin a collection of previous appearances of that character.
don't go crazy and maintain a focus - identify what matters to you about your books (i.e. grade, color, spine, cut, or just plain readability) and stay with that - if you don't you will find yourself starting and re-starting your collection over and over again spending a lot of money.
understand going in that this will take time - which is the best part about it.
as someone said it before, if it seems to good to be true (especially on ebay) it is! I'm amazed at some of the deals on the threads sometimes though.
as someone told me, give yourself a couple of years in before you start buying entire collections (I've had fun buying bulk and selling of the dups - but it requires a separate commitment, plus a whole bunch of room).
and lastly, for now, take care of your books - it's easy to do, whether you use bags and boards or mylars - it's well worth keeping up on it. you let it go for a bit and you find yourself ruining your books...