What does it mean when someone say "you have attitude"?

Have a look at sense 6 of attitude in the OED, with a few quotations from there.

a. Aggressive or uncooperative behaviour; a resentful or antagonistic manner. In phrs. to cop an attitude , to give attitude , etc., to assume such a manner. slang (orig. U.S.).

1985 N.Y. Times 26 Oct. 31/4 If I'm out there for months with everybody yelling at me, I'm going to cop an attitude.

1990 L. Lane & N. L. Andrews Malibu 90265 ii. 18 No wonder the saleswoman had an attitude... A zero had just dropped off the end of her commission.

1991 Athlon's Baseball '91 IV. 25/1 Bonds developed what is called an attitude. Underneath it all he is a nice kid.

b. Hence, any highly independent or individual outlook, approach, appearance, etc.; self-possession; style, swagger, front; esp. in with (an) attitude . slang (orig. U.S.).

... 1990 Police Rev. 28 Sept. 1916/1 In this job, you gotta have attitude, hang loose, ready for anything.

1992 Face Feb. 44/1 The not-entirely-unattractive cast—spearheaded by Jason Priestley and Luke Perry as hunks with not much attitude Brandon and Dylan—set a good few pulses racing and hogged the covers of the nation's teen press.

You have the uncomplimentary assessment (sense a: uncooperative behaviour), which gave rise to the positive assessment (sense b: self-possession). Both are in use, and context is crucial, although my sense is that we are more likely to say 'an attitude' for sense a.