If you would like to work on an issue and if it has no assignees assign it to you and work on it. If you would like to work on an issue already assigned to someone please communicate clearly with the assignees before putting any working in it.
When you want to work on an issue assigned to you, make sure you work on a branch dedicated to that issue. When you fixed an issue create a pull request for your changes to get reviewed and at the end to be merged into the master branch.