Starting Outside, what do i need?

Hi there,
I have access to this very cool greenhouse; but unsure how to start seeds out here. I live in Zone 6a in NY.
Just wanted to start some veggies and flowers outside.
Last frost date Is around May 18.
I am totally new to outdoor growing from seed. I usually start mine in my basement with lights; but they ever do great and I end up buying a bunch of transplants.
Would I need mats to keep it warm enough in the greenhouse? Any other heat sources?
Anything to ensure humidity levels?

Thanks for any insights!