Seriously - pixel art is an incredibly complicated medium. Until you can do regular mediums competantly you havent got a chance at pixel--art. As helm said, we can only help you with pixel-art techniques, as i doubt no-one would be prepared to spend hours teaching you the basics.
We can help you dither, AA, selout, color choice, texture, pallette restrictions, etc...
We cannot teach you how to draw accurately, get a good sense of anatomy, lightsource, build an image with a good pose/flow.
Pixel-art is all about manipulating pixels and playing-god with colors to make an image to the human eye - not about being to lazy to get off the computer and practising your sketching/basic art skills.
About this piece - try and draw individual leaves finalredemption.com is a great inspiration for anyone learning to pixel trees. Also try make the trunk a decent trunk color, make it thicker, give it a proper lightsource, go and look at the color of trees around your house - I wood (excuse the pun
) bet 100$ none of them look like this.
EDIT: damn you ptoing for summarising my main points in far less words