Hi There

Hey, I am a german student. I want to learn everything about online marketing.

I created several niche sites, facebook pages and Instagram accounts.

This is a rather vague question.
You want to learn Online Marketing but what aspect of Online marketing do you intend to learn?
you must learn initially almost all the basics first.
Start with common jargon; SEO, SEM, SMO what do stand for, what is their importance, so and so forth
then after learning how all this works, you have to determine which one of these would you want to learn in detail!
You do not want to be one of those who intend to be "Jack of all Trades"
Trust me on that!

And subsequently, go read this thread by @Sherb CLICK HERE

Pick a niche and target audience, attract views/engagement that will translate into sales for you. Grow the business, then sell it or keep it if you like where its going.