to play online yuh need to have a Gold membership. on the xbox live homescreen yuh could sign up through one of the special deals, but it means giving your cc info to Microsoft and they will do automatic renewals at the full retail price unless you call and specifically ask them to turn it off.
as an alternative, you can buy a 3 month or 1 year subscription card at BestBuy, Amazon, Buy.com etc. The card has a scratch off thing in the back with a code that you can use to renew the service from the xbox. Regular retail is $59.99 for the one year, and about $25 for the 3 month.
Right now Walmart.com has the 1 year for $40
If yuh have patience, regularly scan the electronics forums at Fatwallet.com or Slickdeals.com and they will alert you to sales and other deals. A couple months back I saw a coupon on Fatwallet and picked up a 1 year card for about $35 at BestBuy.
The cards go on sale regularly, so if yuh miss a deal, hold strain and another will come around.