here is a simple 2d example, You didn't say 2d or 3d, but the same code would work for both.
This would also need a timer so you would not be able to continuously jump.
Graphics 640,480,0,2
SetBuffer BackBuffer()
gravity# = 1 ; set strength of gravity
;set initial position and velocities
playerXpos# = 320
playerYpos# = 240
playerXVel# = 0
PlayerYVel# = 0
While Not KeyHit (1)
;get player Input
playerXvel = playerXvel -(KeyDown(203)-KeyDown(205)) ; add user movement to the X velocity
playerYvel = playerYvel + gravity ; add gravity to the Y velocity
If KeyHit(57) playerYvel = -20 ; if the space bar is hit, then jump
;add velocities to the current positions
playerXpos = playerXpos + PlayerXvel
playerYpos = playerYpos +PlayerYvel
; bounds checking
If playerYpos > 400 playerYpos = 400 ;ground plane
If playerxpos < 0 playerXpos = 0 ;left edge
If playerxpos > 630 playerXpos = 630 ;right edge
;draw the screen
Rect playerXpos,playerYpos,10,10 ; draw the player
Line 0,410,640,410 ; draw the ground
Text 0,0," use left/right arrows to move, space to jump"
playerXvel = playerXvel * .9 ; create drag for the X movement
Flip
Cls
Wend