I have a lot of experience with these cars. They can be tricky, especially if you're just starting out. The clutch is very snappy, and at least mine with a new clutch and fresh fluid had a very high engagement point.
The stock WRX engine plus the AWD means the car really bogs if you let the clutch out too fast at idle. My advice until you get more confident, rev it up to ~2000 rpms and then slowly let the clutch out. When you feel the car start to move, try to hold the clutch there, then let it out even slower.
When I say slow I'm talking probably 5 seconds of consistent travel, moving the clutch from all the way on the floor to all the way up. Keep that up until you get more comfortable with it, then try it with less and less throttle, or rolling on the throttle as you roll off the clutch. You will probably still stall occasionally, even when driving around, for a while. Don't worry about it, just clutch in, start it up, and go on your way.
The clutch connects the engine to the wheels, so if the engine is running and the car isn't moving, to get the car started something has to slip. On cars with more torque the engine will keep plowing along, and on cars with less grip the tires will slip some too, which makes it easier. You've got a lot of drivetrain drag, a lot of grip, and not much torque, which means you have to do pretty much all of it with the clutch. So it's best at first to give yourself some help with more revs.
For a while, if you've got a problem, step on the clutch. If you're taking your foot off the gas, step on the clutch. Keep practicing. And don't forget you're driving, don't get so absorbed in the footwork you don't look at where you're going.
There's every chance none of the advice you'll get is the advice that will help you, so you're probably on your own. But this is how your grandma drove, so it can't be that hard.