OK, I think the best thing to do is to have a couple of cycles so you can let your cycle get into a rhythm and you can then have a better idea of what is going on.
It should go something like this:
Day 1 AF
Random period of time that differs from person to person.
Ovulation
10 - 16 days
Day 1 again.
The time between your period starting and your ovulation can be anything really. It depends on your body. This can even vary from cycle to cycle. However, the time from ovulation to your next period will be something from 10 - 16 days. This is rarely differs by more than a day.
So this is the tricky bit. Ovulation is something of a movable feast.
One option is to have sex every other day from one period to the next, that way you can be sure of hitting the right time.
However that is not always possible, so there are ways to detect ovulation.
The most popular are the cheap OPKs you can buy on Amazon or ebay. They are a dipstick type test where you need to wee in a pot then dip the stick in. For these to be positive the test line has to be darker that the control line. The downside to these is they can be tricky to use, weeing in a pot, needs to be done in the middle of the day; and tricky to read. However they are very cheap, about £5 for 30.
The other option is the Clear Blue Fertility Monitor. These take all the guess work out of it and you use your first wee of the day, much more practical. However they are much more expensive. £100 for the monitor, although you can get them on ebay for £30, and £17 a month for the sticks.
What a lot of women do along side these options is to chart. This involves using something like the Fertility Friend website to chart your cycles, intercourse and temperature. Tracking your temperature tells you when you have ovulated, however it tells you after the event, not before. So it is best used with OPKs or the monitor. Here is my chart so you can see how it works.
TBH I would start by charting your period and sex for the first couple of months before you start using anything else. This will give you an idea of how your cycles work. Then move on to OPKs etc. Who knows, with any luck you might not need them.