The best place to start after you have drawn the basic shapes is the trunk. Start at the base of your tree, the roots, and work your way up. Add any detail to the bark along the way such as growths, holes, grooves, etc. When you reach the canopy, don't draw the branches yet. Instead, just mark out where the main limbs begin with two lines with a gap between them. The only exception for now are the lower branches, you can draw them a little bit further until they begin to spread into thinner branches.
If you want to add other trees surrounding your tree, then this is how to do it, otherwise skip these stages to the colouring stage. First draw a line for the horizon just a quarter way up from the base of the trunk. Then add your trees in (which should be simple line structures). The trees closer to your main tree are drawn below the horizon line, while trees further away are above or on the line. For foreground trees, draw a line representing side of a tree trunk partially overlapping parts of your main tree's branches and shade in and add any details such as growths and pertruding branches, etc.
Redraw in pen, shading in the ground and any background vegitation in the process.
Colour in your tree and background. Here, the trees bark have a light coating of brown and applying green over the top of it. I went a bit impressionistic with the leaves. As it is autumn, I scribbled in patches of orange, green and yellow and overlapped each colour slightly. Then I used my pen to scribble in some texture and to make the impression of very thin branches. I used my pen on the ground too to create texture.
And that is how I draw trees. Of course every tree is different, but the drawing technique is the same. Some trees have blotchy bark, but this effect is simple enough to do by shading differently to other parts of the bark. The four seasons will give you more reasons to draw trees, as a diciduous tree will appear differently with each season. Evergreen trees, such as pines and yews, won't change too much throughout the year, but can also be great to draw too. So get out there, kick up some leaves and draw some trees near you!