Just level with your points into the Ret tree. You can heal dungeons just fine while leveling with Ret spec and healer gear.
As for people not inviting Ret/Prot paladins, it's kinda true for raids, but not for lv 60 dungeons. Prot Paladins are actually quite good in a dungeon setting, but because of mana issues and the lack of a taunt, we're never tanks in raids. For Ret, our damage in dungeons is just fine, I often see Paladins at the top, both while leveling and at 60. In raids we can't use our Judgement of the Crusader, which does hurt our damage, but we still bring respectable damage to a raid. Also, we're the only spec that will really need those 2H Weapons, although Fury Warriors might want some of them. Just tell them to STFU and dual wield.
As for the community's idea of Retribution, our DPS is a ton stronger than people think it is, but that's because people think it does 0 damage. Ret damage isn't as high as Warrior/Warlock/Rogue, but we bring really solid utility, and even allow Holy Paladins to drop the 5 points they have in Improved Blessing of Might to put somewhere else (if they PvP, they will love you for this). We bring an extra Lay on Hands and BoP, we buff all of the Melee so the Healadins don't have to, and we make sure that those 2H Strength drops don't go to waste. The problem is convincing a raid team to take you, because as I said, people think our damage is awful, when they are just following the status quo, when they actually don't know shit. Pointing out that you are freeing every Holy Paladin up 5 talent points is a good starting point.
Play what you want, honestly. Is Holy a better spec than Retri? Yeah, it is, but that doesn't mean Retri isn't a viable raid dps spec.
EDIT: As for talents I would recommend going like this, then you have the option to rush into the Holy tree to grab Consecration, or keep putting points in Retri for Vengeance and Repentance.
http://db.vanillagaming.org/?talent#sZZgf0tbcoThis is the spec I personally use
http://db.vanillagaming.org/?talent#sE0zZVuZef0tbcqo