I would advise strongly against getting any single "online" mentor and instead urge you into keeping all your interactions in that regard in the open and from multiple sources.

In the real world I would advise going to a public access dungeon where members can be vetted or club for hands on training in a safe environment and or perhaps attending a few munches.

There are also a number of wonderful non-fiction books on the subject of how to be a good dom too...like "The Loving Dominant" which you can order online or at any adult self help section of Barnes and Noble.