First Aid is one of the easiest to level, yet extremely vital secondary skill. Its ability to offer a free heal in the heat of the battle makes it more or less required for any character who plans on raiding. Its utility carries over 5-man dungeons, and even questing - especially if you are not a healing class.
Leveling First Aid is simple. All of the bandages you will be able to make require only a few cloths of the appropriate level, and can, with a few exceptions, be learned at a trainer. This has some downsides if you've chosen Tailoring as one of your primary professions, but leveling First Aid is not that big of a setback, so it's worth the delay in your Tailoring advancement.
Once you have created your bandage(s), all you need to do is use it on yourself or one of your allies and channel the 8-second heal. A single bandage if level appropriate to yours will usually heal for about 20-30% of your health, but will interrupt its effect if you take damage or move. In addition, players can only be healed with one bandage every minute due to the Recently Bandaged debuff placed on them as soon as a bandage's ticks.
The original design of this secondary skill included anti-venoms such as Powerful Anti-Venom, but the idea was quickly abandoned. Still, if you plan on going back and catching up on old content, those may come in handy.