Hi! I would like to render a simple pine tree. I have some questions. First of all: Should I use billboarding? So is the tree a simple quad always facing the viewer? Second: I need a pine tree texture, but I have not found a good one yet, after searching the net for an hour. Third: Some part of the tree will be transparent, should I use alpha-testing for this? Thanks: Lutyo

1)

Billboarding is probably the simplest way to go

2)

Find a picture of a pine tree using google. Use GIMP to cut out the parts you want to be transparent. Save as TGA.

3)

Yes, use alpha testing w/ the TGA texture.

Depending on the level of realism you hope to achieve, you may also want to consider making pine trees as follows:
a simple cone for the trunk, and and stacks of cones (un-capped) representing the leaves, with the radius of the cones decreasing the closer they are to the top of the tree. You can disable back-face culling for the leaf cones and apply an alpha-blended texture to them. The benefit of this approach is that you can create trees with random hights, widths, and leaf-cones.
Here is an example of the kind of pine tree I have described: link Sorry, I couldn't find a better picture. But you can see how the trees are really just cones (with some branches added to their sides, which, in my example, would be non-existent).
As far as textures, you can probably create some nice bark textures by altering photos you find in google's image search. Just search for "pine tree bark", and you should come up with a great deal of photos. You can then cut and paste a portion of the picture and tile it to make a texture. Creating a pine needle texture will be more difficult, but you may achieve the desired results by using the stamp tool in Photoshop (or an equivalent tool in another image-editing program), on a picture of some pine needles.
Anyway, good luck with your trees.