Why isnt federal taxes being taken out of my paycheck?

I work around 35 hours a week and earn 7.25 an hour. I fill married filling jointly and have three deductions with no child care costs or other deductions. I noticed on my first check that federal with hold was at zero and when i asked why my supervisor told me she didnt make enough to have to pay in and she earns more than i do. my question is this, where is it written what you have to make in order to owe federal taxes? iv looked all over the irs website with to no avail. another supervisor suggested i refill out my w2 for no deductions so i will pay in federal and get a bigger refund but i really cant afford to loose that little extra from every pay check. when the irs is too busy and my employer is ill informed where do i find answers???