I will be brutally honest with you: don't do this. It never really gets easier - you just trade one problem for another as they get older. Any efforts you make will likely go unnoticed, and every mistake you make will be scrutinized and criticized. You are too young for this responsibility, and your first experience with chid raising should be with your own kids, because you will love them even when they aren't perfect, and they will teach you patience, which is vital for dealing with other people's children. Dealing with other people's children requires about 10x the amount of patience you will need for your own kids, because children from broken homes often have a lot of justifiable anger to deal with, and therefore are more prone to problematic behavior.
And yeah - I've been in your shoes, exactly.
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?