Su.Ki. gave you excellent information, (in fact I don't know why she doesn't teach as she has quite a following here on italki!!!); however, I would also suggest that you read ALL the information on the teaching support page before you start to make sure that you understand everything. This is a very rewarding job, and I have learned so much from my students. I look forward to each session and seeing my students' growth over time, but it does take a while to get a good reputation.  Understanding the hows and whys of the site will make it easier for you when you are first starting out.  Good luck!

http://support.italki.com/hc/en-us/categories/200405038-Teaching-on-italki

I don't teach on this site, but I hang around here enough to see how it works.

Here's the easy part:

http://classic.italki.com/teacher/apply/step1

You'll need to upload your teaching certificates and a nice friendly photo of yourself. You'll also need to create an appealing profile page, ideally with a good quality promo video. Then you'll need to get yourself known in the community - answer questions, take part in discussions, make friends. If you're lucky you'll find some students.