The Front Squat is an amazing exercise that I like to do regularly. It's a bit difficult to do at first, but if you start low enough and work your way up, it's actually not that hard. I started at about 50% of my back squat and worked up to ~75%.
As for technique, I'll try to give you some cues I've learned over time, which have helped me a lot.
It's still a squat...
While you're placing the weight in another position, your back and legs should about do the same thing as with a back squat.
- Knees should still be above your feet, but not over them.
- The weight should still be above your mid-foot.
- You should still go below parallel.
- Your back should still be straight (although more upright).
...but it's also different
Since the front squat uses places the bar in a completely new position, you might want to look out for what your upper body does.
- The weight is resting on your front deltoids, not your chest, your arms or your collarbones. This can hurt at first, depending on how puffy your deltoids are. If you're strangling yourself, you went too far.
- Keep your back upright and straight (again). This prevents dropping the weight and nasty back injuries.
- Keep your elbows up. This is even more important to not fall over. Keep them as high as possible and don't let them sag once you get tired. This is by far the one thing that helped me most, as you'll almost automatically hold your whole body in the right position once the elbows are up.
As for grip:
The bad news is, I'm not sure what stretches would be advisable to get to clean grip, but there're already some question on that topic on this site (I think someone recommended aikido strechtes, go look it up).
The good news, however, is that you can use a 'clean grip' even if you're not flexible enough. For this, one uses straps, like in this video (at the start, but the rest is good, too). Like Thibs says in the video, straps can also be used instead of a 'real' clean grip to prevent elbow strain, but that's personal preference.