What should i do? Bratty 14 year old i look after...?

Okay, you need to discipline. I discipline the kids I baby-sit, and I get respect. Simply say, "You can't go on the computer/watch TV until you do your homework/chores." If she disobeys you the first time, say it again a bit more firmly. If she still isn't doing what you say, tell her father ("she's having some trouble focusing on her homework.").