Hi Sami!
OPKs display two lines, the control line and the test line. During infertile days the test line will be more faint than the ctrl line. One day before ovulation day there's a surge in an hormone and that's what the test is picking up (the so called LH surge). This LH surge will last approximately 1 1/2 - 2 days. So you may test one day and get a positive (test line darker or equal to ctrl line) but there's a chance you'll get a positive the following day as well. Ovulation takes place 24-36 hours after the first positive OPK.
Sometimes you get to predict when the LH surge is about to happen cause you will see the test line fading in from day to day (but still remaining more faint than the ctrl line)
You test once per day and unlike hpt's you shouldn't use morning sample, best time is between 2-8 pm and you should test around the same time every day
Since you don't know your cycle now you should start any day I guess until you get a positive. Any other indication that will help? mild pain? clear CM etc? You might be needing a lot of tests so I would order them online but I haven't bought any that way so I can't advise you on the brand
All that said, I wouldn't rely on OPKs to avoid a pregnancy Sami. You get an indication too close to ovulation and that's risky, you can get pregnant by having sex way before the LH surge...
BUT, you can track down your fertile window and start from there, add a few days before and after ovulation and use protection on those days. Just a thought
Good luck and (since I never got around to say this) congrats on your little man
xx
ps I realized I said a lot more than what you asked for, sorry!