I am a 17-year-old girl. I would like to marry my boyfriend. Is that permitted in Germany?

You can get married to your boyfriend.
Marriage is an official relationship before the law.
A registry office will confirm the relationship.

There are various regulations relating to marriage in Germany.
You are allowed to get married from the age of 18.
Sometimes you are allowed to get married from the age of 16:
But then your partner must be older than 18.
And the family court must agree.

You are only allowed to marry one person.
Multiple marriages are not permitted in Germany.

You are allowed to end your marriage at any time if you are unhappy.
That is called divorce.
You can find more information on this website: http://www.zanzu.de/de/scheidung-0

You are not allowed to marry anyone in your own family.
Even cousins do not marry cousins in Germany.
Children from these marriages can get hereditary diseases.
So it is not a good idea to marry your own relatives.

You do not have to get married!
You can get help if someone wants to force you.

The other option is:
You can get married by means of a religious ritual.
There are many different marriage rituals.
But in Germany these are not official according to the law.