Hi, my understanding of this has been:
The first day of your period is day 1 of your cycle.
The cycle lasts until your next period (normally 26-32 days)
Your most fertile period is usually 14 days before your next period is due
(so if you normally have 28 day cycles, roughly day 14 as this is 14 days before the end of the cycle)
But.... cycle lengths can vary, so an egg might not be released on the exact day each time AND an egg only lives for about 12 hours if it is not fertilised so it is not an exact science and having sex only on the most fertile day is not the most successful method.
The best thing to do is to have sex on the run up to the most fertile day, so that the sperm is already up there when the egg is released.
Sperm can live for 48 hours up there, so it is suggested to have sex every other day rather than every day- this way it gives the man's body 48 hours to make enough sperm but at the same time there is always likely to be some up there alive.
The dreaded 'two week wait' comes about because after the egg has been released, there is not really any point in more sex, you just have to wait till the end of the cycle in 2 weeks to see if you are pregnant.
In practice what this means for me is I have a 28 day cycle most months, so I have sex on day 10, 12, 14, 16 and 18 and then test on day 28.
Good luck trying to conceive.