ford F-150 truck with manual transmission trouble

I have a 2002 F-150 Ford truck with a manual transmission. I try to get it to go into any gear and it won't, I can shut the motor off and then it will go into gear and the truck will then run when I start it back up. When I get on the road it will go into gear and shift most of the time, but it will not go into gear very easily. The clutch seems to have alot of play (not the usual tension). The truck has 45,000 miles on it and I don't think the clutch is bad. Can anybody help me with this problem and how much it would cost to fix it.