It's probably a good idea to post your armory so people can comment better on what and where.
There's a couple of stickies in these forums about gear and different speccs. After the last patch they might be a little obsolete (I know my knowledge of some mechanics has taken a hit), but it's there you should start our search IMO.
In my experience (as a palatank) there are not that many rep items worth your while. Cenarion expedition necklace and timewarden leggings from KoT being the most notable exceptions. KoT rep also allows you to get the head enchant for tanks. Oh, and if you are aldor, the aldor chestpiece too. The cool thing about the aldor is that if you have gold to burn, you can just go to your AH, buy as many Marks as you need and voila, you have the rep to get you your chestpiece.
Depending on how casual you are, I wouldn't really recommend a tanking career though. It takes quite a lot of dedication to have good enough gear to tank anything in Heroic. Of course, once you have such gear, tanking said instances is nearly trivial.
I had an easier time gearing up a holy pala (I have a few palas on different servers). The fact that you can use cloth/leather/mail/plate makes it so much easier too. With the new itemization I expect gearing up a holy pala faster than I found when I geared mine up ... If you go holy, you will need the head enchant which you get in Thrallmar ...
Like I said, post your armory 1st. We need to know your gear / specc.