Quite cute. ^^
Regarding tips, you might play with more shading to help give it depth, although with an anime-type style like this it's less important.

I tried shading a little bit, but I don't really know where to shade and how to shade...

It's like anything else and takes lots of fooling around and practice. Generally you need to pick a direction the light in the picture is coming from and put your highlights on that side and your shadows on the side away from it. Things that should be farther from the camera are also usually darker too, like around the edges of the head, or the neck vs the face. This tricks the eye into thinking they are farther away.

When in doubt, google is your friend; Look at a bunch of other people's drawings and see what they did. There's also a lot of tutorials out there too. Here's one that was near the top and looked reasonably useful: http://toi-chan.deviantart.com/art/TUTORIAL2-Cell-Shading-33578859?qo=1&q=cell+shading+tutorial+boost%3Apopular+age_sigma%3A24h+age_scale%3A5

Anyhow, I think you're off to a good start. The line-art is quite nice, and that's the most important part of making a good drawing.