Jedi Guardian - Force
I play a Jedi Guardian spec'd into the Force tree. This is a tank class but a dps tree. I have been successful tanking up to lvl 21. My general strategies are as follows:
1. Jump First - This can not be stressed enough - Use force leap before party members pull.
The worst pulls and most chaos has occured when I have party members that get over zealous on tough groups and pull before I do. With a taunt that has a 15 second cooldown and the inability to out-dps my party members regaining aggro on a whole group of mobs is very tough.
1a. Use the correct form. You don't generate focus points quickly but you mitigate dmg better.
1b. It may be useful to use your Saber Shield (or whatever you use to block/dodge dmg for a short period) once you jump in. Especially if it's a big group with hard hitting mobs.
2. Immediately use Sweep (and repeatedly when it pops) or whatever your area of affect skill is. This gathers a little extra aggro to keep secondary targets off the healer.
2a. If you think you're just out of range of extra mobs when you're about to use sweep move closer so you maximize your aoe dmg and aggro build with secondary targets.
3. Zoom out so you can see whats happening and pay attention to your healer. Be sure you use your taunt at opportune times as you only have one (at least I do so far).
4. Your job isn't to do dps it's to take dmg for others. Be sure you don't use up all your focus points as you may need to use them when you don't have them to try to pull aggro or leap or sweep or something.
5. Kick (interrupt) your mob when you can. This will reduce a lot of dmg to the group and yourself throughout a fight.
I know some of this (a lot of this) is generic but it has worked for me in SW: TOR.
Additionally, I have done the synthweaving crafting skill along with underworld trading and archeology. So far the crafted gear (heavy armor) has been sufficient to be a valid tank. I've also found some blue and other colored drops that have further increased my viability as a tank.