How do you tighten Shimano disc brakes? I have just brought the Trek Fuel ex8 (on Christmas) and already the back brake is squeaking. Also both the front and back are not very sensitive and are not stopping the bike very well. I've looked on youtube how to tighten them but there not really giving me a direct answer. Any help?
Trek Fuel ex8 - has hydraulic disc brakes.
Hydraulic disc brakes do not require adjusting - they are self-adjusting.
In that, as the pad wears - the distance between pad and disc is adjusted automatically.

The squeaking and squealing of disc brakes is a common complaint.
My first advice would be to 
- bed the pads in
- make sure the discs and pads are clean (small contaminants - especially oil will cause serious braking problems)

Bed the pads in by taking the bicycle up a long hill and coasting back down whilst lightly dragging the brakes. This causes the discs to heat up and bed the pads in. 

Hydraulic disc brakes should have a "snappy" feel to them. If contaminated - they will lose power.
If you have dropped any oil (even the smallest amount) onto the disc caliper - clean the pads and rotor thoroughly. Pads will have to be removed and can be baked (facing down) in the oven on top of grease proof paper.

Hope this helps.