You always have 2-handed equipped at all times, unless you are doing something specific, not related to dps.
Dont worry about opener that much, depending on boss, you can do 1 autoattack, then pop CDs. Or Judgement from range, close gap, CDs and go priority list as you are in range. Not a thing to give too much thought, you can do that when in all other important areas you are doing good. As to what is "priority list", check out some guides. Also more "technical"(important) info there, about talents, stat caps, glyphs, everything you need.
You do not wait for your seal to stack to 5. By that time your trinkets/procs/BL will have gone to waste, at least partially. Burst as you start.
Paladin DPS is really straightforward, since it's just a half of your job.
Better is Agility builds, there are limits and specific items. While you are gearing agility items are pretty nice, but you don't actively pursue them. I'd let other classes that use agility get them, since it's a group effort and you win by playing a lot of raids instead of winning an item in one raid.
You can go in Armory, check out PvE ladder, snoop in to those guilds, look at some retri paladins. Pretty much any guild in first page will do, you will notice a trend in gearing on retri palas there.