How did Geralt become a witcher? And why?
He was abandoned by his mother Visenna and left in Kaer Morhen. Witchers took care of him and begin his training. Why he wanted to become a witcher? Well, he was young and idealistic, so he believed that witchers can be like those knights in a shining armors he read about and be a heroes protecting innocents from vile creatures. Most of it fade away after he killed his first monster (which was a bald guy with a rotten teeths) and damsel he saved first vomit after seeing what he's done and then fainted out of fear. Since then Geralt start changing into cynical outcast we all know and love.