I understand how you feel. I am a main healer too and when I decided to start tanking I was a little worried as well.
Chill out. Tanking isn't any harder than healing, it's just very different and requires some time to get used to, but after a few dungeons you'll be fine. Also, imho there's no optimal tank to start with. More important than choosing the "right" job (but you should think of DRK as an advanced tank, it's the most difficult one to use properly so learn how to tank with PLD or WAR before trying DRK), you should always remember a few principles: a) always remember to update your gear; b) examine the other party members (the healer in particular) at the start of a duty so you can adjust your pace accordingly c) look at the enemy list once every 3 seconds to be sure your keeping aggro (no need for this when you get good enough) and d) don't trust your dps party members, you would think that if you pull a large pack of mobs all dps will be doing their aoe rotation but no, there's always that one dps that will just keep using the single target rotation even with 10 mobs around him, so always target the same target of the dps members (ofc a good dps will attack your target from the start, but there are a lot of bad ones).