You need to track your ovulation and have intercourse during your fertile days.
No, it's not as simple as that. YOu don't automatically become pregnant just by having sex. Like the above poster states - you need to track your cycles and pinpoint when you ovulate (using OPKS is most accurate) then you need to time sex leading to ovulation (your fertile days)
Even by having sex on your fertile days it doesn't mean you WILL fall pregnant. It can take some couples years to get pregnant naturally, even by having sex on the fertile days.