Basically, you have to have 40 Social Security credits. You get 1 credit for each $1,220 of earnings (that's the amount in 2015; it will probably go up in future years) and can earn up to 4 credits per year. So you need to work 10 years or more to get the 40 credits. Once you have 40 credits, the Social Security Administration looks at your earnings history, takes the 35 years with the highest earnings, and calculates your benefits based on those 35 years using a formula too complex to explain here. If possible, you want to have at least 35 years with some earnings in order to maximize your benefits.

go visit the social security website and it can do the calculation for you

kj566 is right and so is Bob. Go to the SS website or give them a call and ask them to send you a printout.

It matters how much you earn and for how long you earn. The SS website has info on this.