I wouldn't advise varying the position of your grip, I'm pretty sure none of the pro's do it so it really can't be a good idea. Saying that, as you get better you may find that holding the grip slightly higher up will make it easier to get the ball from low at the back of the court, but you've plenty more to worry about before that.
Consistency is the key to squash. If you can consistently hit the ball tight against the sidewall, for straight drives, and get good width on your crosscourts then you'll be very hard to beat. In terms of length, you need to aim to get the second bounce to land in the nick at the back of the court.
Rather than hitting the ball hard to get a good length, aim it higher up the front wall. It is much more important to get a good, accurate length than to hit it hard. Personally, when hitting from the back of the court I aim to hit the front wall just below the service line, but for beginners I would suggest hitting it higher up the wall.
Footwork is also very important in squash because you're in such a confined space. For the same reason, techninque is also very important. The swing is very easy once you've learnt it. You'll have a lot of trouble hitting from the back of the court if you don't have the right technique. If you're serious about learning to play then you should get some lessons.