Hi I am waffu my last periods where started at 28th december I had/having sex/intercourse since 8th january how can I know either I am pregnant or no?

It's pretty simple. If a woman with regular periods is trying to get pregnant, and her next period doesn't come, then she is likely pregnant. If she starts having early pregnancy symptoms a week or two later, and her period still doesn't come, she's almost surely pregnant. Most women do a home pregnancy test after missing a period (the tests are very accurate after missing a period).