as the above poster said, you can set a ground target by typing /groundset # where number would be the distance
You can also manually set it by hitting the f5 key. This places a groundtarget at your feet. If you continue to hit F5, you can move the groundtarget forwards by hitting the up arrow (move it back with the down arrow). You can also raise it on the z-axis by hitting the S-arrow.
if you mess up your ground target or can't find it, you can always type /groundset 0 to reset it back to your feet.
Practice at it. Properly setting groundtargets is very important to your class. It is not difficult, but then again, it is not very easy in certain situations to get a groundtarget in a tower or keep. There are tricks to it that you will eventually learn, but for now, just start with the basics.