It's always difficult to say for definite in these circumstances, because some people lay the groundwork, then others add bits, and often the last piece of the jigsaw is really quite small ... so who DISCOVERED the thing?
In the end, the Nobel prize was given to Francis Crick, James Watson and Maurice Wilkins, so it is fairly official they it was THEY who 'discovered' DNA.
However, most the the breakthrough work, historians are generally agreed, came from a woman, Rosalind Franklin. Personality failings and a degree of treachery, it must be said, meant, however, that she was beaten to the 'final success' by Crick and Watson.
This article tells you the whole story.
PS Don't forget to say please when you are asking for help.