How do i move from my old friends and make new ones since ive started high school?

i just entered the ninth grade. ive grown up with my best friend nikki ever since we were born and were very close but shes only in 8th grade. and plus she has another best friend ashley whos in the 8th grade too so i figured theyll get along better and plus ive been in too many fights with nikki and her and ashley make me feel left out sometimes and them they can be reallly immature. so ive decided to make friends with ppl my age so how do i drift away from them and make new friends and best friends cause im ready to move on?? thanks:)