I had sex after two days of my periods went off. And I had taken I-Pill birthcontrol table.so I will be helpful to prevent pregenancy.

The only way to prevent pregnancy 100% is abstinence. Since that is not always possible there is the birth control pill, depo provera (injection), implants, condoms.

Honestly the best way to prevent pregnancy is to not have sex but I doubt that is what you are wanting to hear. No method short of sterilization is 100% perfect. There have been some members who have been on the pill and using condoms and still got pregnant. I have also known people who have gotten pregnant while on their period.
The only real thing that you can do is be as safe as possible. Even though none of the measures are perfect it is better then not.

If you took it within the first 72 hours, it should prevent you becoming pregnant. It doesn't work all of the time, but that is what it is meant to do.

As the others have said, use other forms of birth control if you want to prevent getting pregnant. Emergency contraception is not meant for regular use.