Check your Spec and your DPS / Healing

For raids, don't come with a hybrid pvp spec. My best advice is to do an armory search for a super raiding guild that you know of, and look up their guys that have the same roll you do. Then, check a few of them (cause someone maybe doing some pvp) but that should be obvious.

Once you have your spec, check your DPS on the heroic target dummy in any of the major alliance cities. For raiding, a minimum is roughly 1800 dps and I'm not sure what is needed for healing. I guess - don't go OOM over a few minutes? If you do not meet the rough requirements for a raid, then try to run heroics for the week. Within 1 week, you WILL have well over the minimum requirements, if not more.

I started my first Naxx pug at 1700 dps. We barely cleared Patchwerk, and called it on Grobulus cause he mauled us badly. My main issue, I wasn't hit capped, so I had a lot of misses. I ran heroics for 1 week, and I was upto 2k. After 1 more week of heroics, I had everything I needed, was around 2200. I did Naxx once, got a nice drop or 2, and that's where I'm at now. Unbuffed I can pull around 2600 dps I think. So, my best advice, run heroics! We'll help you. It will help Naxx runs go smoother, so we have better runs.

Arastar

Just to expand on that a tad, a healer really should have at the very minimum for 10 man Naxx:

* Around 1500 Bonus Healing
* 160 MP5 (mana regeneration while in combat)