I seriously need some parenting advice!!! HELP

I have a 7 year old little girl who has serious behavior issues. I just had my second child last month and of course my oldest is jealous and acing out. She gets in trouble at school almost everyday and 2 weeks ago she stole her classmate iPod and damaged the screen. I agreed to replace the screen and then the mother of the child called me saying my daughter took inappropriate photos in the iPod. I am so lost for words and completely embarrassed. First she stole and lied about it then she actually used this iPod and took pictures of herself. I DO Not take inappropriate photos in front of my child. She has a tablet and I've never seen any inappropriate photos in there. I need some advice on how to deal and what to do. I can't give her my undivided attention because I have a 1 month old to tend to. This is hard. Bring a single parent is no joke. I don't want to raise fresh little girls. I have to be more strict. She has been on punishment for weeks now and she still acting up in school. What else can I do?