How do I make myself more assertive?

I am seriously struggling with asserting myself and voicing my opinion if it is different to the other persons. As a kid I grew up watching my parents and siblings argue a lot and almost always (even on very silly, trivial matters) it would lead to screaming fights and violent meltdowns. For this reason I am very scared of confrontation or getting on someone’s bad side. I never push for anything, because in my mind the smallest thing could lead to someone becoming very angry and turning against me. Even when I do finally assert myself, I can only say a few sentences before my throat dries up and I start stuttering, and my heart races so fast I feel dizzy. It makes me look and feel like an idiot. I know most people are not like my family, and I am tired of being a pushover and always following what another person wants. So how to I fix this?