I typically follow back cause, I mean, they must've followed me for a reason. So why not? I don't really care about stats and all that (mainly because I don't fully understand it all ๎) so as long as you've got a good reason to follow me, then I'll follow back :3
Typically ignore random <20kcs. I feel like if they really have something to say to me, they'll send me a gift or wall me.
Follow then send a message. Normally a simple "yes ma'am/sir?". Almost all of them don't want anything at all