Can't control my two year old

I'm finding it hard to control my almost 2 and a half year old daughter. She is seriously on another level. I had two boys 15 months apart and both of them put togeather weren't as hard work as my daughter. She doesn't listen to me at all no matter how many times I say something, she will just smirk at me thinking she's funny. If she doesn't get her own way she screams and screams and screams. I try to put her in timeout in her room and she just comes out straight away no matter how many times you keep putting her back in. I've tried smacking her, it doesn't phase her, I just don't know what to do anymore she's diving me insaine. I have a newborn baby and she's constantly all over him and she doesn't know how to be gentle. She's always poking his face and his head and I'm forever telling her not to touch his face or head but nothing I say gets through to her. I feel like I'm at my wits end and I really need some advice on how to turn my terrible twoster into a behaved little girl