I agree with matt that alot of players want ranked team mates these days, but alot of people aren't that bad. If you find yourself wanting to join a group, try pm'ing the leader. Tell him that your unranked but you willing to learn and try, alot of the high end pvp'ers don't mind taking you along, especially because it can often take a few hours to get a good group together. As long as you don't royally screw it up then they won't mind you being there. Another good idea is to start your own group!
Secondly its all about contacts in pvp. Befriending people when you pug is possibly the best way you could advance your HA career =P It makes it much easier to get groups again in the future. A serious pvp guild would be a good start. ( I know shouldn't of said that but its helpful advic.e)
Be thick skinned! this is possibly be the best bit of advice I can give you. pvp'ers can often be the biggest douche's you'll ever meet. If you messed something up, don't worry about it, ignore hate and flames and take the mistake and learn from it, no one plays perfect constantly. Take everything with a pinch of salt, don't be afraid to spew some bile back if people give you a hard time either
About builds and things, alot of people will use team setups from pvx (gimmick builds mostly, like sway) if you manage to wiggle onto a high end team, then you might find yourself having to run some balanced build that isn't actually on pvx, so be prepared and have a large pool of skill unlocks at your disposal. Same goes with guild teams, as most will often have their own setups which they are comfortable with... So not everything relies on pvx.
Necro would be a good class to start off with, to learn the basics of HA. As a necro you will be running a hex spam most of the time, so its a good way to start off and get to grips with maps and tactics vs different teams etc. Ele is also a fairly good place to start where you'll most likely be running a blind spam or savannah heat pressure setup. Both of them are nice and simple. I wouldn't recommend mesmer if you are still getting used to HA.
Lastly, PvP requires alot of communication so vent and TS are pretty much required. Alot of patience will be needed aswell, dont give up because one person won't let you into their team, keep trying there are some nice pvp'ers around still
It will take alot of dedication to break into HA, don't expect to get in and start gaining alot of rank nearly instantly =]
I hope some of this helped
Mainly because I've been thinkin about it and want to try it abit more. As first, I more known in PvE (I know like every GW PvE thing 
) than PvP, though I play alot PvP. I do everything except HA, which means I am a big fail in HA, so thats why I never do it. 
).
