Fisting Kittens -
Okay, there are a million tips you can use when squatting. However, it is best to start safe and easy.
Phrost is correct that powercleans are good but they are also a VERY advanced exercise in which its VERY easy to injury yourself. Its probably better to start with squats and/or deadlifts first for 6+ months befor thinking about it..
Do not do them on a smith machine. I keep telling Phrost this and he keeps ignoring me. :) The reason for this is the smith machine locks you into a specific plane of movement, which isn't how your body is desgined to move. This puts massive structural pressure on your knees, lower back and shoulders - as they have to compensate.
Follow these tips:
1) Because you've built your leg muscles in other exercises, they can carry more weight in the squats than yous shoulders like to. If you've not used to doing squats, your shoulders won't be used to carrying the bar on them. It will be uncomfortable for a while.
2) While your shoulders should be uncomfortable, your neck should NOT be. This is probably because your carrying the bar too high on your shoulders. Try moving it down a bit. If your shoulders are scrunched back (yeah I know thats not really a word...) and holding the bar they actually help to create a bit of a platform for the bar to sit on. An easier idea then a large pad (which may get in the way) is doing squats in a comfortable jumper (ahhh... in the US are they called pullovers?) to provide a little bit of padding.
Experiment with a light bar to see where best to place the bar. It should be just below the big protruding vertebrae at the base of the neck (I can't be bothered looking up its name...). However, placing the bar too low can cause too much forward lean so be careful.
3) There is debate about how low you should go in squats. Its different for everyone, but damage can be cumulative for people who constantly go below 90 degrees. They may notice pain in their knees after years. At least until you've got the movement down, only go to parallel (but use a mirror if possible, because often guys who think they are going parallel are only moving 5-6 inches!
4) Not sure of your dimensions. The taller and lankier you are, the harder it is to maintain good form while squatting. Tall skinny people are structurally better suited to deadlift.