You own everything you’ve published on MEDIUM 100%. You don’t have to get anybody’s permission to repost them or submit or publish them elsewhere and you don’t have to indicate that they were first published here. That being said, most paying markets will NOT run reprints, so do not submit work that has first appeared on Medium do them. (You need to do your research before you submit work to a new editor.) But many markets (like the Funny Times) are happy to run (and pay for) reprints!
You can repost anything that appeared on Medium to your own blog or website. You don’t have to credit Medium.
If you self publish a collection of your own work, you can include anything that has solely run on Medium.