I know this is probably different in every car but how far should my clutch pedal come up before engaging??

also would a slightly worn clutch cause my car to not really pull away properly( sometimes feels like a lack of power)

one other thing how many miles till a clutch would need to be changed??

My driving is fine but dunno about before I bought the car 

1: Clutch biting point does vary from vehicle to vehicle. I've driven some where bite was towards the bottom of the pedal travel and some where it was almost at the top of the pedal travel.
2: yes, a worn clutch won't fully engage so therefore less power and clutch slip
3: Again, this varies and depends on how the vehicle has been used and whether or not previous owners have kept their foot on the clutch when setting off and changing gears (clutch dragging). I've had clutches go at 20k miles and some at 60k+. 

When my clutch started to fail I noticed ithe car would not gain speed on the dual carriageway when trying to go over 40mph. The engine revs went up but the speed stayed at 40. I could pull away in low gear at junctions without any problems. When the new clutch was fitted it felt softer and the bit point was close to the floor.