Asteroid belts are good places to find "rats" or NPC pirates to kill. Not only is it good practice for combat and testing setups, you can also make a few ISK. NPC Pirates have "bounties" on them. You can determine the bounty on a pirate by checking its info. The amount of the bounty depends mostly on the security status of the system you are in. Just like ore the quality of the pirate depends on location and how "hard" the system is. The bounties can vary alot but usually falls between 6,000 in 0.8 and 1,900,000 in 0.0 though pirates in lower security status systems have bigger ships with better equipment. Not only do you get the bounties but you also get items in the form of a lootable wreck. After a pirate has been dispatched it leaves behind a wreck that you can approach and open from within 1,500m. Sometimes the items looted from the wreck are worth more than the bounty.