There are a couple of options - see https://wiki.openstreetmap.org/wiki/Sandbox_for_editing . You can use the dev.openstreetmap.org and edit all you like there, although you’ll only see your changes in the editor; there’s currently no-one rendering that site on a map.

There’s also the “walkthrough” that you get with the iD editor - everybody who signs up to OSM will, by default, be taken through a series of test edits.