Sure i see it now. I see where i should use AA and this really can improve this piece. Also you have right, i use too much light in some places. 

Thanks a lot for help. Soon i want add something bigger and i use your tips for make it looks better. 

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Okay, in terms of light source, exeggutor's left(viewer's right) leg has little to no shadows and/or too much highlights. It's right(viewer's left) most head needs more shadow as it's in the dark according to the light source direction. A few of it's left most leaves may have their highlights removed as they too are in the dark.

Some of the banding issues will solve themselves when your palettes improve, they'll also be fixed when you begin to use AA. When these things don't fix banding, changing the cluster shape might.

Thanks for tips. I agree colours is my weak point at this time and still i learning how compose them better.

If you can tell me in which areas I made the most mistakes I would be very grateful.

Here are some pointers for this piece that may be of some use. The majority of the colors need more contrast. The light source direction is inconsistent in areas. Avoid banding.