You need to buy tabbed batteries. These have solder tabs already welded on them. One source is Digi-Key. They will even build you strings of batteries. You do not want to be welding, or even soldering directly to the battery. Unless you really know what you are doing you can destroy the battery, or maybe worse.
Modern packs often have a charge controller built into them that is matched to the batteries. If so, be sure to buy batteries of the same capacity and chemistry. Buying higher capacity is a waste of money since the controller may not let you fully charge them (this depends on the controller type; getting too techie here).
Don't try to replace NiCd or HiMH with Li ion batteries. Their charging requirements are vastly different, not to mention the voltage. Also, if you have a quick charger (who doesn't, now?) get batteries designed for quick charging.
