When should I use an OPK?

I’m very new at TTC. I had a scare a few months ago and over the past 2 months, my husband and I decided that we shouldn’t be scared to get pregnant, but that we should really try as we are doing well in our lives currently. Well, on my Glow calendar, my days are green with ovulation on Saturday. I could be doing this totally wrong, but I took an ovulation test today (Clearblue) and it said I’m not ovulating, while my glow app says I have a higher chance of getting pregnant today than on Saturday. I don’t get it. Any help is appreciated!