But if I buy Calendula off. then that would be correct?

And what part would I use?

The flowers.

Thank you.

Both Marigolds, Calendula and Tagetes, are edible, culinary, medicinal. Leaves from Calendula can be used in cooking, that's why its another name is "pot marigold". As far as I know, only leaves of Tagetes lucida (Spanish tarragon) are used in cooking. Otherwise, petals from both marigolds are used. Personally, I like to eat the whole flower when I get a chance :)

I use a lot of calendula blossoms for tea. I particularly like to have some dried as part of the blend of herbs I drink for tea this time of year. I also infuse it in oil to use on the skin.