How to join a PUG

Don't.

There is no worse example of humanity than the idiots you will run into in a PUG. 70% of them in fact, are going to have at least one annoying useless person, and as if to add the whip cream on top of the stupid-flavored ice cream, that person will be the one who objects to your very presence. "omg palidins can't dps u noob l2p." This shining example of failing English class will be a response you see more than once. Paladins are only expected to heal any 5-man you join, for reasons of mass stupidity. You can get into a few groups that'll accept you, but I would just as soon avoid the whole hassle.

Ah, but you need gear!

OK, fine.

First keep in mind you have chosen to gimp your group finding by being Retribution. Sacrilegious as it may sound, speccing Holy or Prot while doing 5-mans for gear isn't a terrible idea at all, as you guarantee a group will have at least 1 of the 2 hard-to-find roles, healer or tank.

Failing that, there are three options that will make it easier to PUG. One, wait 'til the "LF1M for XXX" messages pop up. You're living a little dangerously here, as the last class is usually the one they NEED, aka tank, healer or CC, but if they filled all their roles, at this point they'll take nearly anyone, and may be willing to give you a shot.

Two, start the group yourself. This is important because if you gather up people in the list of people looking for that dungeon you're setting the rules yourself, and thus give yourself a chance to get in, since it is your group. This will take awhile since you now have to search for a healer and a tank for the run, but at least you have a shot at putting this together now. It's not a bad idea to casually mention that you'll be DPSing in the run to the first couple of people you invite. If they leave because of it, they're jerks you wouldn't want to run with anyways. Get them out of the way.

Three, bring at least one or two friends. By having people in the run who know you and are friends with you, the "omg noob" idiot who joins will be slightly more reluctant to start crap with you, as doing so means he pisses off 3 people instead. Honestly, run as many groups with friends/guildies as you can get a hold of. It'll save you a lot of heartache in the long run.

Just always keep in mind, idiots are going to be idiots, and you'll have to deal with them. The Commonly Accepted Belief ® is that Ret Specced Paladins are useless, and 90% of your brethren have done nothing to challenge that belief. If they act like they don't trust you in the group, that's fine. They have a pre-conceived notion about you, but as long as they allow you a shot at proving them wrong, you can and will prove them quite wrong indeed.

And every doubter you correct in the capabilities of being Retribution-specced is one more person who's suddenly more willing to give that second Ret Pally a chance in a later run. Every nonbeliever you convert is a believer who helps out a fellow Ret Pally down the road. It may drive you up a freakin' wall, but in the long wrong, it's worth it to push against these boundaries, as every little thing helps us become more accepted in the general WoW culture.