Somebody using my account

As mentioned above, follow the steps in this article, to gain control again:

If you think your Apple ID has been compromised - Apple Support

Changing your Apple ID and password is also linked in the article above.

Change your Apple ID - Apple Support

And after that:

What to do after you change your Apple ID or password - Apple Support

If you have an iPhone 7 plus, and you are getting the message, that someone is using your device, just after you tried to sign into your Apple ID account or iCloud, it is usually you, that triggered this, and the location is the nearest server, that your provider is using.

When I sign into my account, I'm getting the location Frankfurt, although this is 200km away from my actual place.

So, no need to change anything, as long this has been your device.

If not, check this:

If you think your Apple ID has been compromised - Apple Support