How can i find out what bra size i am?

im 16 and im not sure what bra size I am, when I was younger I ust to just have the same size as my sis because shes only a yr older and we were the same size but now were not, its like whenever I buy a bra its either too big or too small, so I just wanted to know how I can find out what size I am, is there a websire or anythink u can put measurments in? xx