You give your projected yearly income, your pay stubs evidencing that you are getting it and 2013 taxes and W-2. If we had to just put the monthly income, very few people would qualify.
This is not legal advice and a client attorney relationship is not created.
You should put your 2013 income with paystubs /2013 tax transcripts /2013 W2s attached to the affidavit.
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Your question shows why having an immigration attorney do these types of forms is a good idea. :) If you have not already filed your 2013 taxes/received your tax returns, write the estimate of your annual income for 2013, rather than the actual income for the few months you have been actually paid. If that amount does not meet the guidelines, you will need a co-sponsor.
This advice does not form an attorney-client relationship and is merely informative. It should not by itself be relied upon to address a legal concern.
You'll need to base your income on your last filed tax return. You should file your 2013 tax return before filing the I-864 application unless you earned enough in 2012. Disclosing your 2013 W-2 wages with the 2012 tax return/transcript can confuse USCIS and the National Visa Center. It can trigger what is called a "kickback letter." Good luck.
This is general information, not legal advice, and does not create an attorney client relationship.