Hello, @AxelBreitung!
Please, read first this basic info about staking: Understanding the basics of staking - how and why, and even when
Your profit will be proportional to the amount of coins that you put as a stake. This mean that the more coins you stake - the more profit you will receive.
I don’t really understand your question, what do you mean “how many coins I should buy”?
Staking will be possible with almost any amount of coins. If you have 10 ADA - you will probably be able to stake 10 ADA. But you will receive proportional profit - maybe something around 1 ADA per year.
There’s no official tutorials yet, because staking in Cardano is not yet available. It is in development right now and will be fully available somewhere closer to the end of the year (Q3-Q4 2018). Once it is available - there will be plenty of tutorials and videos and manuals.