You can't. It's physiologically impossible.

Why would you want to lose your memory?

Ok, what is it you would like to forget ?

zale9droid does not find this helpful : I want to know

This is inappropriate. I asked you a question. In order to answer your question, you have to answer mine first. We are not mind readers here. If you want someone like that, contact the Psychic Network!

If you want temporary amnesia (not amnishia), you will have to have severe blunt force trauma to the head. When and if that happens, it is possible that you could become permanently brain damaged as well.

Your family doctor can easily take care of this for you, Just go in and explain this to him what you want and why. He will be sure it is taken care of.