pitching is done face to face so you'd have to make contact with somebody at HBO just to get the opportunity. for newbies the best (and sometimes only) route to pitching is at pitchfests. many production companies send reps to these fests and there is no litmus test prior to pitching so anyone can pitch for a fee. unfortunately, you have to go to them so a trip to LA is almost eminent. sometimes they go to NYC or Chicago or some other major city.
HBO may or may not send a rep to any particular pitchfest so i suggest you NOT limit yourself to them. there are several other cable channels who produce their own series for the small screen: showtime, usa networks, TNT, sci fi channel, bravo, etc... take your pick and/or pick them all. if they attend a pitchfest then why not?!
also, check out the websites for cable channels. most will give you information on how to submit ideas.
also - try entering your idea in a contest for a tv pilots. if it places well, then you have much more clout when it comes to begging for the opportunity.