I haven't even touched the door to fixing cars.
I've never heard of the expression "to touch the door" although I can guess its meaning because of the context. You could say, "I haven't even started fixing doors." Otherwise, these terms will be more familiar with AmEng speakers—
A rookie (AmEng) "I'm a rookie when it comes to fixing cars."
A novice "I've only just started learning the basics of car mechanics. You could say I'm a novice."
Fixing cars is just like learning Chinese to me.
Not an idiomatic expression, but it's fine if you want to say that learning something is virtually impossible. However, I don't think that's your intention, you have started learning something but you're at the very beginning. Hence, I would suggest the following;
To learn the ropes "I'm far from being a car mechanic, I've only begun learning the ropes."
To get the hang "Learning how to fix cars takes time. I've only started to get the hang of it"
To master "It will be some time before I master the art of car mechanics"
I am a green hand at fixing cars
(Which is sort of OK, in the sense that we understand what you're saying.) It's better just to say green.
To be green lacking training; not trained; inexperienced. "I'm still green when it comes to fixing cars"