Welcome to Scratch, Herblist2!
you might want to check these forum-threads:
http://scratch.mit.edu/forums/viewtopic.php?pid=47089#p47089
http://scratch.mit.edu/forums/viewtopic.php?pid=46887#p46887
http://scratch.mit.edu/forums/viewtopic.php?pid=46200#p46200
they also point to some example projects and should give you some ideas how to get started...
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This is quite simple:
Simply put the following command on the sprite you wish to jump:
<when[ Space ]key pressed>
<change y by(5)>
<wait( 1 )secsc>
<change y by(-5)>
If you want to make it smoother, then do this:
<when[ Space ]key pressed>
<repeat(5)
<change y by(1)>
<wait( 0.2 )secsc>
<change y by(-1)>
It gives the same result, but looks better
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