There are, eventually, six companions available for each class (and potentially a seventh). A Melee tank, a ranged tank, a melee DPS, a ranged DPS, and a healer -- and the droid that comes with your ship.
Each of them has two stances you can pick between (except the droid); for the healer, you can choose between maximizing DPS or heals. The ~ key (or a button on the UI near the companion portrait) will let you look at all the skills available. You can right click to toggle their usage on or off, or left click to have them use the skill on your target.
So, to make a long story short, you can get your companion to heal you or itself by targeting you or itself and pressing one of the heals in its bar, provided it is a healing-type. (At this point, as a Jedi Knight, that's probably just the ship droid -- which does no DPS and so is likely a bad choice.)
Otherwise, the default AI will have the healer companion heal you or itself automatically -- you'll want to keep aggro off of it as much as possible so that it doesn't waste all its heals on its squishy self.
As a Jedi Knight: T7 is a Ranged Tank. If you've gone tank, you'll probably want to turn off most of his tanking abilities and put him in DPS stance, until you can replace him with Kira (Melee DPS); if you've gone DPS, you'll want to make sure you keep his armor updated and might want to keep him around a bit longer (although I'd still turn off his grapple); you can still manually activate his taunt if you need him to pull something off of you.
You will also want to use your Meditate ability to heal both you and your companion between fights.