Trees

How do you make a tree . I just rented some land, and want to create full trees to act as a privacy fence. I know there is the tree control in the build menu, but it just seems to random choose from a fixed set. Do I have to build my own using prims?

Trees are objects.
Look in your inventory.
Also try the Freebie places, like Yadni's for boxes filled with plants.

You can also make your own. Try the Design and TExture forum for tips. But its basically 2 prims standing upright...with a tree texture and alpha around the tree. (look down from above on a tree, and you'll see what I mean)

Each tree takes 2 or 3 Prims. (as does a bush or a clump of flowers). Be careful, or you'll have garden and no prims left for furniture

If you've got a long wall to cover you might want to look for one of those 10m long 'privacy screens'. A HUGE picture that you place on the edge of your property to hide the neighbors. A giant jungle screen, with a few Prim Trees in front can be very effective, and use far fewer prims. (For some ideas of what I mean, goto the SL Exchange, enter the Marketplace, and search on Privacy Screen.)
http://www.slexchange.com/

The advantage of rezzing trees from the library in your inventory is that they each count as a single prim. There is a pretty good variety of trees and shrubs available.

I agree with the others -- if your goal is simply to put trees on your land, you should buy some and put them there. There's lots of really nice ones in world. If you want to build a tree from scratch, you'll need to learn about transparency and flexible prims. I have an example on my blog (www.secondlifehowto.com) of how I built a field of wheat using these principles.