This build guide lists the commonly-sought talents that paladin players want. In parentheses following, it states the number of talent points you must consume in the respective tree, in order to get that talent maxed out. No matter what build you choose, you should know what paladin abilities use which crit type.
Also, in chooseing talents, you should keep in mind where your own strengths and weaknesses tend to be. If you keep running out of mana, and you choose a holy build, than improved blessing of wisdom might be a good choice, for instance.
Or maybe just some more +int gear =P
Many people see a holy build as mostly a healer build. That's partialy true; a holy build can also be a good damage build, if you pick up divine favor and holy shock, maybe even seal of command, and can gather, say, +200-300 spellpower through items and abilities. All in all, holy is a very flexable tree to spec into. If you do decide to take a holy build, be sure you look at +spellpower coefficients
Holy is seen as the best raiding tree, but it does not mean that a retribution paladin can not be a good raiding paladin. In holy there is holy power, illumination, divine favor, and lasting judgment. All of these are great healing talents, but they have more to do with mana than anything. The only talent in holy that really directly effects the ammount healed is Holy Light, which is only a 12% bonus. Bringing a flash of light from around 300 health to around 900 is completely gear; +spellpower and +healing specificaly.
(see this page discussion)
Protection Paladins are excellent at PvP with Reckonbombing, and Improved Hammer of Justice, along with Blessing of Kings(Paladins are divided on this. A lot of people like it, and a lot think it is a second or third rate blessing, depending on class) and Seal of Command. If you store 4 Reckoning charges, use Blessing of Kings and Seal of Command, you can easily OHKO anyone in cloth or leather armor.
Paladin tanking is an interesting thing, and speccing full protection to tank may not be the best thing to do. This is because with improved righteous fury, divine favor, and holy shock, you can put out almost 1000 threat instantly without any sort of +spellpower bonus. This is a substantial ammount, and works almost like a warrior taunt. But with improved righteous fury and Holy Shield, you get a 90% bonus to threat just from improved righteous fury, and holy shield will do some 273 threat per proc. The difference is, a holy build might have a little more trouble holding aggro than a protection build, but a holy build could grab initial aggro quicker.
A retribution Paladin's most powerful talent is Seal of Command. Most paladins have this ability, as it is useful to everyone, since it scales with both +AP and +spellpower. Typicaly, retribution and holy will be taken together in varying degrees since the two trees compliment eachother wonderfully. Particularly, improved judgment is useful to eaither a holy or retribution heavy build.
One question that gets asked a lot is "Should I take divine favor or repentance?" Well, the answer depends first on which ability you like more. Second, it depends on if you plan to mostly PvP or PvE. Repentance is awsome in PvP, but because it only works on humanoids, it is not so useful in PvE, where divine favor is much better.