Overcoming Social Anxiety: How Do I Get Started?

my therapist says to overcome social anxiety i should practice talking to people, and start with things as small as saying hello to someone i walk past in the school hallway. i know exactly what i should do in social situations but something just stops me. i look at a person i want to talk to, i have the exact phrase i want to say, i know logically they won't judge me, but i never actually go through with it. i think "just do it, it'll be fine", i'm telling my legs to get up and walk over, but it's like a dark force is holding me back. i don't know where to begin with overcoming social anxiety if i can't even get the first step right. how do i work my way up to it?