How do you open, and keep the conversation going?

So I've been socially withdrawn throughout most of my life (not by force, but by anxiety) but I always felt like I'm actually an extrovert just stuck in a loner bullshit, like I actually do feel good talking to people but anxiety and adrenaline annoys the shit out of me and also my inability to start a conversation. Like I just can't think of something that someone who I don't know when speaking can start conversation with about something we both can talk about. Like except the introductory part, at which I'm also bad because I ain't know what I need to not tell and what I have to, I suck more at pretty much anything else.