Could you tell me the meaning of "go up"? Is it different from just "go"?
In this case, to "go up to [person]" is an informal expression meaning to go and approach, or to go forward to. It is a little bit different from "go" and sounds more natural when speaking. Charlie Brown here is stating that he had always wanted to approach the little brown haired girl and speak with her. Ex: Go up to that man and ask for directions. Ex: It is difficult for many people to go up to strangers and start a conversation.