Im an h2b visa worker, I work in the state of Colorado and I would like to know what is the porcentage of taxes that we should have to pay and also I would like to ask, why in the company that I work for, they made us to put in the w2 form that we were single even I was married.

Thanks a lot

It appears that you must learn more about tax filing. To give a proper answer, tell us:
a. When did you enter the U.S. and the type of visa?
b. When did you start on H1-B visa?
c. Is your spouse in the U.S. Working?

You can certainly file as resident for tax purpose as Married Filing Jointly after you meet the requirements. You will get standard deduction of $10,700 and two exemptions of $3,400 each for 2007.

Also, if you meet the requirements, then on your W4 you can get 3 or 4 allowance as married.

The company is playing CYA to make sure enough taxes are withheld to cover your tax liability, just in case you have to depart for your home country suddenly and unexpectedly.