starting email marketing for a tiny shop, what do i send first?

Before any of that: only add people who actually signed up. Importing everyone from your order history is a grey area depending on where you are, and a bought list is the fastest way into the spam folder. Also do the domain authentication your email platform walks you through (SPF and DKIM records) and send from an address on your own domain, not a gmail one. Gmail and Yahoo got a lot stricter about this, unauthenticated mail ends up in spam far more often now.

a welcome email. before any newsletter or sale blast, set up the one that goes out automatically when someone signs up. it gets opened far more than anything else you'll send, and it keeps working while you're busy with the shop. keep it simple: thank them, say what the shop is and who's behind it, point them at two or three best sellers, and if you promised a discount for signing up, put the code right at the top. once that's live, add an abandoned cart email if your platform supports it. after that a plain monthly email is plenty to start with.