Like anything in life, you get what you pay for. Ratter is right, suspension, particularly rear, is a waste of time on a cheap bike, just adds weight and looks daft. Get the lightest frame you can afford, and if possible puncture-resistant tyres, e.g. Armadillo. Carry something to get you home in the event of a puncture, e.g. SealFlate at just �6. A puncture 20 miles from home is very depressing !
Keep tyres really well inflated, e.g. up to 80psi on a mountain bike. So many people ride round with underinflated tyres, they are harder to pedal and more prone to punctures.
Bikes are a way of life for me, again I would say invest as much as possible and go for the lightest frame you can afford. It will become your friend and companion. Have fun, and don't be ashamed to get off and push up the steepest of hills.