I've seen a few people create some trees in 3D which all had the same problem. The easy way to create a tree would be just to take a box, and start doing some extrutions on the top face, and then extrude out some branches. The problem with what I usually see is that they have the main trunk going all the way to the top. That would be good only for an evergreen tree. For a normal tree however, I've put together this small tutorial.
First I will start off with a simple box and convert it to an editable mesh. If you have 3DS Max4 then go with the editable poly since they tend to make a better mesh in the end.
In the polygon sub-object mode select the top polygon and lets start extruding. The trick is to have a shorter trunk. It doesn't go all the way to the top. I usually like to divide my trunk in two and then go from there. The first division of the trunk can be any amount you want.
Remember that its a 3D tree so when extruding faces, don't work in one viewport. Give your tree some depth.
When you have extruded the faces you want to create the shape of your main branches, add a meshsmooth to see what your final tree will look like.
As you can see the bottom of the trunk looks kind of silly. Lets work on the roots a bit. First I select the bottom face of my tree and extrude down twice to create this look. That will stop the bottom from being rounded out like it was before.
Now I take the side faces of our original box and extrude those out to make the roots. its always good to have your landscape turned on so you can see how the roots will look resting on the surface. In this case im just using a box to be my landscape.