When you start depends on how long your cycles are - if they're around 28 days, I would start testing on CD 10 - once a day in the afternoon until the test line starts to get darker and then twice a day, maybe late morning then late afternoon. It is considered positive when the test line is as dark as or darker than the control line. It's best to hold your pee for a few hours before testing and don't drink anything during the 'hold time' so it is a concentrated as possible.
hope that helps! good luck with them!