The sentences you quote could mean quite a few things:
You did it:
1) You did it! [Hurrah! You succeeded!]
2) You did it. [You are the one who did it (whatever it was, good or bad).
3) We have been looking and looking for the one who commited the crime. All the evidence tells us that you did it. [You are the one responsible for commiting the crime.]
4) Many people have tried to improve the morale in the office, but, with your cheerful attitude and good suggestions, you did it. [You succeeded where others failed.]
You've done it:
1) You've done it! [After much hard work, you are the one who has succeeded at a difficult task.]
2) Now you've done it! [Oh, dear! You have really made a mess of things.]
It all depends on the context...