what speed does an F18 need to reach to take off the ground?

Answer 1

about 240 knots.

FWIW, an F14's takeoff speed was 239.25 feet/sec.

Answer 2

T/O IA/S (indicated airspeed) is a bit more complex than Bekki has implied by a single IA/S. Check this out:

"The takeoff speed required varies according to factors such as air density, aircraft gross weight, and aircraft configuration (flap and/or slat position, as applicable). Air density, in turn, is affected by factors such as field elevation and air temperature. This relationship between temperature, altitude, and air density can be expressed as a density altitude, or the altitude in the International Standard Atmosphere at which the air density would be equal to the actual air density." [See source.]

Before any professional pilot takes off, she will check the runway temperature and elevation because that affects how long a roll the aircraft will need to reach T/O IA/S. And, of course, A/C weight is also folded into the T/O length equation.