Is this a healthy weight?

hey im 14, weigh 7.5 stone and i'm 5'8 is this a healthy weight because different websites tell you different things. i have been told that i am dangerously underweight, that i am fine and also that im over weight. i think im over weight and would like to go on a diet, but my mum says i'm a heathy weight. so i dont know what to do.