Generally speaking, you can file a tax return for 2011 as soon as you have received all your relevant tax documents, such as Form W-2 reporting wage income, Forms 1099 reporting interest, dividends and other types of income, and so forth.
Do be aware that the Internal Revenue Service starts accepting electronically filed tax returns on January 17, 2012. You can mail in a hand-prepared tax return earlier, but the IRS generally starts processing hand-prepared returns around the same time (mid-January).
Also be aware, that you cannot file a tax return using only your last paystub. There's additional information found on the Form W-2 that is needed for your tax return.