You'll be upset if you do badly, ecstatic if you do much better than you thought you would, grudgingly happy if you do better than expected, or, like me, slightly dissapointed / unemotive if you did slightly worse than you could've but not badly.
My parents seemed to care a lot more about it than I did, and I observed the same with my friends.
Something which may affect how you view the experience: it's common for sixth forms to not let you take a subject at A-level unless you get an A* in the relevant GCSE. Not sure how it'd work in yours, but bear in mind.