You don't pay from your pocket once you receive the grant money. Grants are a type of funding that one receives to pay for the college education and are never repaid. The only thing you pay is for your upkeep and maintenance. Read more on http://www.scholarships.com/financial-aid/grants/.
Be very careful that this is not a scam. Most grants will not require you to pay anything out-of-pocket at the beginning of the grant. There are grants known as matching grants, which require that you put money equal to the amount given by the grant, but only over a period of time. Not usually at the start of the grant. I am a former grants administrator, BTW.