what do you have so far? if you can show us some code, we could help a little more. if you don't know where to begin here is basically what you would do (this is a form of gravity and then how to implement jumping based on that).
1.create an array that has enough slots for every object that will be moving. this is your verticle speed variable.
2.in your main loop, put a for next loop for 1 to 65535(the max number of objects you can have)
3. inside the for next loop, have it check if the object(that is equal to the variable used in the for next loop(example below)) exists and then go to a subroutine to handle gravity if it does.
NOTE: everything in the subroutine that has to do with certain objects must be done using the variable in your for next loop. ex
for i=1 to 65535
if object exist(i)=1
gosub grav
endif
next i
grav:
move object i,500000
return
obviously, this isn't what you want code wise but is gives you an idea of what the structure should look like.
4.check for collision with the object and your landscape.
5.if it isn't, decrease that objects spot in the array you created earlier by whatever number you are using.(ex if the object is #5 then use the array slot 5 and decrease it by 1 if that is your gravity value)this can be done using the variable from your for next loop.
6.now, the actual JUMPING part. in your main loop, check to see if the player pressed the jump button then(if they did) go to a jump subroutine.
7.this subroutine should be fairly small. it checks for collision between the player and the terrain, and if they are colliding, set the array slot for the player(you should use the same slot# as the object's#) to a high number, say 20(experiment and see what works)
these steps make it seem more complicated than it really is but try it. i didn't give you the code because it helps if you figure out how to do most of it on your own.
