I'd say write a script explaining your channel, and why I should subscribe. What makes you different, what makes you stand out? Do some simple yet professional looking editing with footage from your other videos. Maybe something like,
"I am Anna, here's who I am, more words describing my channel and videos, and here's just a short sentence about how I make awesome videos on a regular basis!"
--cut to shots of the beginning of your vids, playing literally three seconds, just enough for titles to be said, showing how you cover a broad range of topics and something to surely spike the viewers interest--
"I hope you've found something you'd like to watch, blah blah blah, call-to-action for subscription."
Something like that, maybe?