Use solder wire of an appropriate alloy. The most common solder alloy used in electronics is 60% tin and 40% lead, sometimes notated as 60/40. This alloy is recommended if you are new to soldering, though it is somewhat hazardous, requiring proper ventilation, breathing protection, or a soldering iron with a vacuum attachment.
Various lead-free alloys are becoming popular recently. These require higher soldering temperatures and do not "wet" as well as Tin-Lead alloys. However they are safer and can be more effective. 96.5 % tin to 3.5% silver is the most successful and will produce a joint with less electrical resistance than any tin-lead alloy.
Both lead and lead-free formulations are available online at places like solderdirect.com and in your local Radioshack or Home Depot store.