I wrote this post a while back about witnesses. https://steemit.com/steemit-guides/@pfunk/a-full-steemit-user-s-guide-to-steem-witnesses
There are also these posts by others:
https://steemit.com/witness-category/@someguy123/seriously-what-is-a-witness-why-should-i-care-how-do-i-become-one-answer
https://steemit.com/steem/@steemitguide/steemitguide-what-is-a-exactly-is-a-steem-witness-and-why-every-user-should-vote
steemit.chat runs Rocket.Chat which has its own apps. Search for the Rocket.Chat app in the app stores.
Thank you @pfunk! :) I've also voted for you as a witness.