Take a pair of scissors and place in your hand. Open device and sharply close device to properly operate. Top however much you want bro. It's just gonna grow back. Your plant is not a one shot chance at getting marijuana. It's a never ending bucket of chances to obtain fruit, for as long as the plant remains alive anyway.

Once you cut the top growth the plant will be forced to grow outward. No science, no art, just geometry I guess. Wait, that's a science.

T5's would be a good light to start with in your space. When you learn how to prune properly you will find that 24" is enough space to keep clones and mothers in. It's a little cramped for flowering if you ask me, but it can be done. T5 lighting will cause much less vertical stretching during the veg cycle. Look into something called cab growing and you will find lots of people in your shoes.