The command line is probably the hardest option, but the instructions for it are here:
https://github.com/steemit/steem/blob/master/doc/building.md

The APIs are explained here:
https://developers.steem.io/

The developers.steem.io also has a lot of other tools available for accessing the API.

If you lost access to steemit.com though, your first option probably wouldn't be to go directly to an API. It would be a lot easier to find an alternative site/tool that you could use instead, like Vessel, SteemConnect, or Busy.org (just to list a few).