S.R.
Best advice, get over your hard time with the discipline. :) I know she's your only girl and she's the baby, but you do NOT want a spoiled teen in your future.
My girls have both hit the "terrible twos" a bit early... right at 18 months. It's like a little switch flips and all of a sudden you have this monster on your hands. :) For hitting, I hold their hands so they can't hit and say "we don't hit" or "no hitting" or something to that effect.
Tantrums are best ignored so long as they're not hurting themselves or anyone else. If they're going to hurt themselves, put them in a safe place where they can tantrum out and ignore it.
If the "no's" are in response to something she's supposed to be doing (like helping pick up toys or something) then you can tell her one or two more times, then FORCE her to do it. In the picking up toys example, you can take her hand, close her fist around a toy, and release her hand once it's over the toy box. I had to do that with my older one, we'll see what happens with this younger one...
*sigh* The progeny of two stubborn adults is stubborn children, in case anyone was wondering... :)