A good idea would be to post your next pixel on the forums as a WIP. You can learn a lot from people trying to help!

So you're hoping to make pixel art illustrations and the like? That's even closer to "regular" painting than many other kinds of pixel art, so don't neglect to study the general principles in addition to pixel art specifics. What makes Fool's art great is that he combines a keen understanding of general art principles (form, colour, light, composition) with strong pixel art techniques (clean lines, good colour reuse, appropriate AA). You will need to study these things too. Fortunately, there are many resources around! Few combine both pixel art and illustration, but there's no need for that. General tutorials/articles/resources will do just as well, and the principles of pixel art can be and have been adequately explained in isolation.

Thanks for this advice! I will try it :)

do you know fool? One day I would like to make pixel art as good as him ^^' 

just for me as a hobby (and for practicing patience ^^')

Well, what are your overall goals? What do you plan to do with pixel art once you get better? What you do for practice will depend on your goals. There's not much sense in, for example, doing a lot of isometric lineart practice if you don't want to do isometric art.

One thing that any pixel artist can benefit from, though, is doing shape, form, and material studies. Make some circles of various sizes (hand-pixel them to practice avoiding jagged lines), and make spheres and plates out of them. What makes spheres look round, what makes plats look flat or concave? Try making some cubes and cones too, with simple shading just to show their 3D form. Creating simple forms is a basic exercise in drawing and painting, and it's just as important for pixel art. When you've become confident with forms, then you should move on to studying colour.

Thank you very much for your feedback! I'll try to keep it easy next time :)
To be honest, I find it very difficult where to start ^^'

I think it's a bit odd that you have the outline on the bottom in a lighter colour than the shadows around it. It's entirely plausible to have the bottom be a lighter colour due to reflected light, but that doesn't seem to be the case here. Plus, if you're starting out, it's probably better to stick to simple lighting anyway.

As temping as "simple" objects like rocks are, I think they're poor practice for pixel art when you're just starting. They're much more challenging to pull off well than they seem because they're very sensitive to careful use of colours to look good. I recommend trying something with well-defined colours of its own for your next practice piece, so you can focus on pixelling techniques and colour theory/interaction.