How do you tighten Shimano disc brakes?
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.