I Haven't exceeded the free tier limit still its showing bill

It must be because you have used some paid services like a NAT gateway or an elastic IP left unattached!

If you log into the AWS Console and go to Cost Explorer you'll be able to investigate exactly what services the charges are for. Then you can delete the resources you have in the console for those services.

How old is your account? If it is older than 12 months then free tier limits no longer apply and you will be billed for all usage.