I dont have my PIN yet but I need to buy some textbooks.

Can I due at walmart???????

Of course you have to follow the "rules" to make having a cash back card work for you. What are these? Don't buy more than what you'd buy otherwise, pay off your card every month, and so on. But I'm going to assume that you're more advanced than that, and thus I'll move right on to the meat of this post.
When accepting payment by credit card, merchants typically pay a percentage of the transaction amount in commission to their bank or merchant services provider. Many credit card issuers, particularly those in the United Kingdom and United States, share the commission with the card holder by giving the card holder points, airmiles or a monetary amount. This last benefit, a monetary amount, is usually known as cashback, although it is often separated into two words (cash back) in the United States. Where a card issuer operates such a cashback scheme, card holders typically receive between 0.5% and 2% of their net expenditure (purchases minus refunds) as an annual rebate, which is either credited to the credit card account or paid to the card holder separately, for example by cheque.
Rewards based credit card products like cash back are more beneficial to consumers who pay their credit card statement off every month. Rewards based products generally have higher APRs. If you don't pay your balance in full every month the extra interest you would be paying will eclipse any rewards you earned.