Begin by drawing a rectangle and separating it into strips - most posts are made of multiple planks bound together, like a barrel, so these strips represent the separate planks. As these lines must be dark and neat, draw them before you do any shading so that you can correct any mistakes with ease. Now shade the whole shape. It doesn't need to be particularly smooth, but a more-or-less consistent shade, as shown.
To draw the correct texture of wood, make long pencil strokes up and down the paper in the same direction with the flat side of the pencil led. This step shows the sort of pencil-work you should be aiming for. Practise on a separate sheet of paper before you try drawing it on your artwork.
Use the technique described in the step above to draw the texture onto the post.
This step is optional. I decided to smooth the shading/texture out a bit by using cotton pads/tissues - this removes some sharp edges and makes the texture a bit more uniform - but leaving it rough also produces a nice effect. It depends on what sort of texture you are looking for.
Now add finer lines to the texture. Using a similar technique to the one used in Step 3, draw lines up and down the post with the fine point of a sharp 2B pencil. These lines must be straight, so draw them slowly and draw the short - the longer the line, the more likely it will start bending. Don't use a ruler though - the lines can't be perfectly straight or it won't look natural. Again, practise on a separate sheet of paper beforehand.
You may have wondered what was in the upper left corner of the drawing. It is an owl. See my tutorial on "How to Draw an Owl" if you want to learn how to draw this owl.