How can I earn the Cleanup badge? It is written that "first rollback" but I don't know how to rollback. Can somebody please tell me how to rollback and what to rollback?
"Rollback" means rolling a post back to a previous version of itself. You can do this by looking in the revision history of any post and clicking the "rollback" button on an earlier revision.
Note that since you have <2k rep and therefore can't edit other people's posts without approval, you'll only be able to do this on your own posts. (I just checked on a site where I have <2k rep.)
See also What is a 'rollback'? from the main meta FAQ.
As with pretty much any site ability, you should only roll back when it's actually appropriate to do so. Please don't roll back perfectly good edits to your posts just in order to get the Cleanup badge.