Opks confusion

Helaina • Married in October 2015 and ttc since the month before the wedding (no patience lol). Off depo for 2 years.
I'm a bit lost! We've been ttc only 2 months now so early days but I really don't understand opks. I never get a positive! Had a very faint line once but that's it and I've tested first thing and in afternoon. My period is usually mid month so I start testing week after but nothing.