Hey augustin 
First of all I want to congratulate you for starting from the very beginning. Many beginners don't realize how immensely important those "simple" exercises are. Skipping that part for other more exciting studies, only leads to inefficient understanding of form and light. And we don't want that, right? We want to be awesome!
Anyway, you have done a very good job for a beginner. Here are a few pointers:
1) The main lit side's value (right plane) is extremely accurate, so if you didn't color pick it you should reward yourself with a cookie 
. The top and the left planes' values are off and that is because you made the background darker than the original. What does the background have anything to do with it? I'm glad you asked.
You see our eyes perceive forms and depth through visual contrast, so when two colors are next to each other and they are similar in saturation and/or value our eyes will readjust them for better visual clarity.
For example the squares A & B below are the same color. Welcome to the wacky world of painting
So by making the background darker, you are led to make the top plane's value relative to that and you end up with a darker top plane. And since you have a darker top plane, you make the decision to make the left plane even darker to compensate. It's like when you try to solve a mathematical problem and you get a single number wrong and that derails the rest of the equation. Don't worry though, it's something that you will constantly face, so it's good that your are learning about it early in your artistic journey. That way you can be aware of it next time.
2) Your perspective seems off. Next time please do not neglect to use it, if you didn't.
3) On the left plane you rendered the reflected light from the floor. Nice catch! I would suggest to make the smaller shadow on the bottom far left corner more diffuse. It's created by the reflection of the background, so it should be fuzzy. And there is a tiny reflection on the bottom far right corner of the right plane.
That's it! I recommend for you to do another study with my suggestions in mind and after that a new one with 2 cubes next to each other, one bigger than the other. If you need any clarification or extra help just send me a message.
Keep drawing!
