Thank you very much for your help triplefox :)
It's so obvious I didn't thought to give a look at the examples first. I've found how to deal properly with slopes at least ^^ There's a little problem left though: Maybe it's because Godot uses the Physics engine to manage collisions and movements but when my player moves up along a slope, he jumps a little bit after reaching the top of it. I guess it's because of the momentum of something like that. It's not very a nuisance for the moment but I'll probably need to find another way to correct this...
I still need to learn the engine, but again thank you for your help ;)