As others have already said, the official date of the analog to digital switchover is February 17, 2009.
Go to www.dtv2009.gov to see where you can get a $40.00 voucher coupon so that you can purchase a DTV Converter Box. When your coupons arrive, you can then take them to any store that sells these converter boxes and redeem them on the box that you like. Most of these boxes retail between $40.00 and $70.00. Most of them cost about $60.00. The coupon will cover the majority of the cost and then your out of pocket expense is only $20.00.
These boxes are widely available all over (mass market stores and specialists stores alike).
If you have cable or satellite dish, then you are not affected and you don't need to do anything. The switchover mainly affects anyone who DOESN'T have cable or dish and solely receive their TV signals through a rabbit ears antenna.