Hey Nat
Right depends on the type of OPk you have - i bought clearblue digital smiley face which i loved and it came with 7 sticks. If you are unsure of your cycle length or it varies month to month then it might be worth going on ebay (or i think you can buy them from a link at the top of the ttc forum) and they are like litmus paper, really cheap and they will show a line.
Basically, if you are temping then temp every day, first thing in the morning before you get up. You can record it on fertility friend and you can also record you CM and your CP on this too - as well as when you get your period etc. If you get a dark line that matches the control line then you can mark it as a positive opk and that there is a good chance you are about to or that you are ovulating. They tend to detect the surge prior to ovulation 24-36 hours
How regular is your cycle hun?xx