You say your hair is like string--are you saying it's thin? Or does it look dry and kind of worn out? You don't give enough info here. I have fine hair, a lot of it, and it's pretty straight. When I want a better look, I use clip in hair extensions which add bulk as well as shine. But if your hair is overprocessed and looks dry and worn out, a serious conditioning pack is the answer.
I mix olive oil, almond oil, and coconut oil together and put it on my hair from root to tip and rub it in really well on my skull. I usually use it warm (not burning hot) because the heat opens everything up for the oil. The oils replace whats lost when you shower and the rubbing increases blood flow and helps your hair grow. I cover it with a bag and a warm towel and leave it on anywhere from half an hour to all night. The longer its on your hair the better it'll look and feel when you wash it off. When I wash it out I shampoo once or twice (depending on how much oil I put on) and use conditioner like usual. I only do this once a month but depending on how bad your hair is you could do it like once a week until it gets better and then only like once every month. The oils are all really cheap and have lasted me two years now and sometimes if I have really dry skin I rub almond oil on it and its gone. They dont cause zits because the cells are to big to clog up skin. Ohhhh and you smell really good after because of the coconut :) hope I helped