Unless you know when you ovulated, you just go by your lmp (40 weeks from day 1 of your last period) Just remember that it's only an estimated due date. Then as you get further along in your pregnancy and get ultrasound(s) they can either change it or leave it as is.
I agree with the others. Google due date calculators, unless you know your O date. If your cycle is normally longer than 28 days, ADD that number of days to the internet due date calculator based on your LMP. Subtract if your cycle is usually less than 28 days. If you know your O date, use a calculator that is based on O or use that date instead of LMP and subtract two weeks.
Re: How to tell when you are do
You can google "pregnancy due date calculator" and get several places that will tell you based on the first day your LMP