Hogwarts is generally careful about teaching this kind of powerful magic. However, based on the knowledge I've been able to collect, this should prove most useful for us who wish to become talented wizards and witches very early on in our lives.

Through the art of Legilimency, you will be able to explore the mind of another. I am, myself, not knowledgable about Legilimency enough to even write about it. I do, however, possess information on how to block it out: through the art oc Occlumency.

First, clear your mind. This is very important. Clear your mind of every thought and feeling. This way, any Legimiens trying to attack your mind won't find anything to steal from it. Remember that Occlumency is a very difficult and rare skill; It requires tremendous will power. Repeat this practice often, and with time you will learn how to shield against attacks. It will, however, take many years, possibly decades, before you can master this skill: If you intend to do so.