How to become more confident ?

Known as the shy person it sucks :/ i get really angry but im not confident enough to show that side in front of mates and teachers n stuff so like yeah i need to become more confident too, everyone says im not confident but personally i think im rather a gobby cow :) Soo please help cus i have to be able to actually be heard ? Also trying to make new mates at school with people i wouldnt normal be friends with how am i suppose to start talking to them because i dont think they like me too much as they are really the 'populaires' and im tagged as a rebel :) Thanks :) xx