Working with Child Themes

I’m new to using Child Themes and really just have some basic questions. I hear that when you mess with the styling of a theme, that styling can be overwritten if you have not created a new directory for the theme. Is this true?
Also, I’ve created a new directory, copied my style.css file and have been editing that file. I have also switched my theme from Twenty Eleven to Twenty Eleven Child (I named it as such). If I make any changes to the .php files in the parent directory, can those files be overwritten in an update? Should I just copy EVERYTHING from the Twenty Eleven theme directory to the Child one and just rewrite those files as I wish.
Sorry if it seems like a N00b questions, but I want to make sure I got this right before I proceed. Thanks.