Hello @Marco Gerhardt - and welcome to the community!
In cloud there's a Priority field you can use. The options here are maintained by your Jira administrator and the options are available for all projects.
As far as determining the priority, that's up to you and your business process. You can certainly use the priority field to determine that. Additionally, you can drag issues up and down - with higher ranked issues reflecting a higher priority for your team.
Hope this helps!