If you haven't tanked before and are looking to start on a serious level there is nothing as valuable as running with friends/guildies to start with. Especially if one of your friends/guildies is already a good pally tank so they can 'see' things you're doing or respond to questions you have.
There are 4 key elements to succesfully starting to tank - there are many more than that to become a great tank ;-)
1. You need to control the pace. If you let the DPS push you, you'll get yourself into trouble. Mark kill priority - skull and X, so you're not trying to taunt 4 mods running at 4 different people. Controlling the pace will be key.
2. Know your threat abilities. Even if you're not great at using them, know which abilities you should start with in AoE and single-target rotations. At first you may not be 'optimal' in using them, but so often I've run with new (thinking CTA only) tanks that only seem to know about 2 or 3 abilities to create threat.
3. Know your defensive CDs. Knowing how long they last, how long they will take to be ready again, this will go really far into making you 'easy to heal' as you can mitigate the big damage spikes healers hate.
4. Know the fights. Know when the damage is coming. Know where the tank needs to place them, no when you need to kite the boss through the molten lava, etc.
I know all 4 sounds like duh! things - but if you're good with these 4, you'll be in good shape. If you start with guildies and/or friends, you'll be in even better shape. Basically, if you know these 4 things, it may be rough at times, but it won't be a confidence killing wipefest. If you don't know these things you'll have a harder time beating the fight and that's not good for your confidence.