re: focus -- yes, that's key. but of course, it's bad to be blindly focused down a bad direction, too. don't put too much pressure on yourself early on, since nearly everyone flops and flounders.
re: what to do over the summer -- i don't know if there's much you can do to prepare :) maybe reading your professor's past papers? again, i wouldn't stress about it too much.
re: focus -- yes, that's key. but of course, it's bad to be blindly focused down a bad direction, too. don't put too much pressure on yourself early on, since nearly everyone flops and flounders.
re: what to do over the summer -- i don't know if there's much you can do to prepare :) maybe reading your professor's past papers? again, i wouldn't stress about it too much.