So I am now at the fun part where I start the qualitative interviews and reach out to people (or institutions). Unfortunately, other than following them on twitter, I have no previous contact with these potential interviewees and am not sure what to say in the first email. Any tips?

It depends - have you got the info about your study on your official uni website? I would put something up.

Which sampling method are you using? Convenience? Snowball?

If you already know these people over Twitter, I would probably informally contact to see if they want to take part, and maybe link to the said web page for more info? I would be more likely to respond if someone informally asked me first, feels more personal than receiving a mass email.

Or you could get a gatekeeper to recruit for you. So ask your professor/supervisor to email on your behalf. It really depends on who you are trying to recruit.