Translation:I bought flowers for my girlfriend.
Almost good :) "pieniędzy", because you have the negation (even if it's 'musisz' what's negated an not 'wydawać' specifically).
Plus this "ty" is so redundant that it really sounds weird in such a phrase, strange emphasis. But don't take it wrong, that was a really good job :)
'dla' takes Genitive.
And obviously 'moja dziewczyna' is Nominative, so it works almost only as the subject of the sentence...
Why not "...for my girl"? I think it is valid in English to call your girlfriend like that, correct me if I'm wrong