Cut the main tip if you want to top it, give about three inches, cut on a 45-60' angle above the node (where branches come out with fan leaves) but dont cut the mini future branches off near the stem. make a few slices with a clean razor blade and dip in purple clonex, i leave it in the clonex for at least 10 seconds. Make sure you have your wet jiffy pellets ready first before cutting anything.
You can cut the main tip or cut lower branches if they are more than 2mm thick and enough length but the more you cut the less branches and colas you will have in the end. if you have the time a plant can coniually give you branches to cut off every two weeks or three weeks.
Just make sure there are no future branches cut off and your mother plant will keep on producing branches/clones.
You can trim leaves, as water transpires out of the leaves but you must have some leaves left and the middle/top leaves are usually facing vertical not getting any light, so cut the tips of the sideways leaves. just a bit so you have about 3 or 4 inches of leaf under the light.
Instead of clonex there is a clone cutting powder at hardware shops you could use instead but clonex works for me.