As you probably know, Ret Paladin DPS isn't all that great at first. But the good news is, when you get to higher levels, you really get a lot of DPS boosts. The first step to keeping high DPS is generating Holy Power, which is obtained by using either Hammer of the Righteous/Crusader Strike, Exorcism, or Judgement. When you reach three Holy Power, use Templar's Verdict to majorly damage your target. Or if you're having to face 4 or more targets in one basic area, use Divine Storm instead. The most important ability would be Exorcism on cooldown or when Art of War occurs, as Exorcism does a fair amount of damage (At 70, it takes down over half of a common quest enemy's health). Lastly, always use Hammer of Wrath on cooldown when the target is below 20%, or when your Avenging Wrath is active. This is more important than Exorcism.
The main buff to must keep applied to you and your raid is Blessings of Kings (At level 81+, replace with Blessings of Might). This is always the most important buff to keep applied to your group. However, this is not the most important to keep applied to yourself. You'll want to always, ALWAYS keep Seal of Truth active (For AoE, Seal of Righteousness), as this provides a major damage increase, and allows you to use your Judgement ability. Once hitting 72 and 75, you get your highest damage bonus for boss fights, Avenging Wrath and Guardian of Ancient Kings. As I just stated, use these on strong enemies such as bosses. The last buff you must attain to yourself is Inquisition, which you must keep applied at 3 Holy Power, and refresh it every 23-30 seconds (at 3 Holy Power). This will upgrade your overall Holy Damage and Critical Strike chance, although Critical Strike does not help Paladins that much. As for the debuffs, the only thing you need to keep on your target is Physical Vulnerability (Judgement) and Censure (Seal of Truth DoT), but if you use Hammer of the Righteous, you may be able to keep Weakened Blows applied, weakening physical damage the target deals. But beware Forbearance, a debuff placed on targets affected by Lay on Hands, Hand of Protection or Divine Shield that prevents them from having those abilities used on them again for 60 seconds.
A lot of Ret Paladins can also come in handy by giving the healers a hand with a few strong heals, either on yourself or on another target. Ret Paladins, though, only have 3 heals (Excluding your three 90 talents). You have the basic Word of Glory first, which uses 1-3 Holy Power to heal yourself with, and is best used at 3 Holy Power obviously. The next is your basic Flash Heal, which you can keep at a 3 stacks of Selfless Healer by using Judgement as often as you can for a quick, free-of-mana heal (explained in talents). Your last one is Lay on Hands, the strongest heal you can have. Use it ONLY when it is really needed, because the 10 minute cooldown can be annoying.
"Hands" are the Paladin-specific abilities that can provide a large amount of utility for you and your group. My personal favorite is Hand of Freedom, which will clear you of snare or root effects for a short time, similar to Emancipate without a lasting period. Next is Hand of Protection, which for 10 seconds will protect your target from all physical damage. This does NOT include magic attacks, ranged attacks (from hunter-like targets), and already-applied bleeding and poison debuffs. Another one of your Hands is the Hand of Salvation, which will make the target lose all threat for a short time, best used on targets gaining aggro that they shouldn't have. Your last Hand is Hand of Sacrifice, which will clear all harmful magic effects off the target and transfer 30% of the damage to you. Cannot self-cast. Your other abilities are Divine Protection, which will decrease the magic damage dealt to you by 40% (20% magic, 20% physical with glyph). Devotion Aura will keep your allies from being silenced and interrupted, as well as decrease magic damage taken by 20%.
Stuns, Interrupts and CCs
Your main stun is Hammer of Wrath, which will stun the target for 4 seconds (or with the level 30 talent Fist of Justice, 6 seconds with half the normal cooldown). Your get two CCs (excluding talent), Blinding Light at level 87 and Turn Evil at level 46. You'll want to use Blinding Light to give you a moment to heal yourself when taken damage, but do not attack or the CC will break. Your other CC, Turn Evil, isn't the best. It will make an Undead, Aberration or Demon flee for up to 40 seconds. This does not include the Forsaken Horde race. Your interrupt, simple as any other, may only be used while the target is casting, and they won't be able to cast for another 4 seconds.
Level 15: Long Arm of the Law for a speed boost with Judgement OR Pursuit of Justice to give a passive speed boost, but decreases when Holy Power is used.
Level 30: Burden of Guilt, good in slowing your enemies without sacrificing damage by using Seal of Justice and only be at melee range to slow them OR Fist of Justice to make you use your stun more often and stun them for 133% of the time.
Level 45: Selfless Healer, keeps a good, strong and fast heal in your pocket when you need it!
Level 60: Clemency, will allow you to pop your "Hands" twice as often OR Unbreakable Spirit to make you use your Forbearance abilities more often!
Level 75: Divine Purpose will give you a chance for some serious damage or healing. When you start doing raids at max level, use Sanctified Wrath or Avenging Wrath to increase your DPS by a lot for a period of time.
Level 90: Execution Sentence will do a great DoT or HoT when you need it. For PvP use Holy Prism to damage a target to heal nearby others, or heal a friend to damage nearby enemies.
Major Glyphs: Templars Verdict (Always keep active, survival boost). Double Jeopardy (Damage boost when switching targets. Keep active). Mass Exorcism (Good AoE in melee fights. Change when needed). Harsh Words (Gives you something to use Holy Power on other than Inquisition in ranged fights. Replace when needed) Divine Storm (Survival boost, can AoE to self heal. Replace if needed)
Minor Glyphs: Righteous Retreat (Hearth faster, or even get away from PvP combat quickly). Falling Avenger (Reduces falling damage when Avenging Wrath is active)
Strength > Hit (7.5%) > Expertise (7.5%) > Haste > Mastery > Critical Strike
Don't pay heed to the stats until you hit 90!
NOTE: Watch out when using Hammer of the Righteous when questing, the AoE can accidently pull more than you want sometimes.
This is how I usually play my class, I hope this helps! :D