Tree shape

[Rapture]

If you mean a 3D model, just take some real-life pictures for inspiration.

Examples...

Tree 1 (3d model)
Tree 2 (2d Art, with some nice branches)
Tree 3 (Good trunk view)
Tree 4 (Very good)

If you want a decent looking tree trunk (Up to 4k tris if you go crazy retopolgy; But 1-1.5k would be a good estimate if you do it carefully; Without leaves.) spend a lot of time sculpting. Get the curves, bumps and everything. (Tree Picture 4 would be a good example.)

But if you want a really quick dirty one, do something like Amnesia's.

/////////
I did a crappy one a long time ago, and I went overboard with retopology. But here is my attempt, I need to do another soon...

My Tree