How much is my social security benefits reduced if I am younger than 65 and earn wages ?

If you are under full retirement age for the entire year, SSA deducts $1 from your benefit payments for every $2 you earn above the annual limit. For 2013, that limit is $15,120. In the year you reach full retirement age, SSA deducts $1 in benefits for every $3 you earn above a different limit, but we only count earnings before the month you reach your full retirement age. If you will reach full retirement age in 2013, the limit on your earnings for the months before full retirement age is $40,080. (If you were born in 1943 - 1954, your full retirement age is 66 years.).
If you were born in 1960 or later your full retirement age is 67.
Starting with the month you reach full retirement age, you can get your benefits with no limit on your earnings.