How can I become assertive without fearing retaliation?

I've always been a passive-aggressive person due to my negative upbringing. I was never taught how to feel good about myself or how to stand up to people in an assertive way. I've grown tired of being fearful that if I speak up then someone will retaliate against me. I do speak up when necessary, but I don't feel that I speak up the right way. I don't know how and what to say and I feel that my self-esteem is lacking. I want to be able to be bold and confident as well with no apologies for being who I am. If anyone that has gone through this can answer my question I'd really appreciate it.