http://www.cra-arc.gc.ca/bnfts/wtb/fq_qlfyng-eng.html
The working income tax benefit (WITB) is a refundable tax credit intended to provide tax relief for eligible working low-income individuals and families who are already in the workforce and to encourage other Canadians to enter the workforce.
You can claim the WITB on line 453 of your income tax and benefit return if your working income is over $3,000, and you meet all the eligibility criteria.