Definitely buy LOTS of them - you can get them on Amazon for dirt cheap. I think I got 25 for $5 or something like that. If you have no idea when you ovulate, I think they recommend you start on CD10 twice a day (morning and evening or afternoon and evening) and continue until you get a positive result. With OPKs, there will always be two lines so a "positive" is determined if the test line is equal to or greater than the control line. These are mine from last month.
https://i132.photobucket.com/albums/q28/emmiesanimalrescue/My%20Pictures/Family/Baby/Cycle1OPKs.jpg
I also chart my BBT each month so I had an idea of when I was going to ovulate and only had to use these two all month but that's not the case for everyone. So based on the OPKs and my temps, I ovulated on CD15. We unfortunately didn't catch the egg that month but there's always this month!