What are the best tactics on Modern Warfare 2 so I can get a tactical nuke (video games) ?

HELP! I'm not the best at awarding killstreaks on Modern Warfare 2 so please could someone help and tell me tactics on getting a tactical nuke (25 kills) or at least a chopper gunner or harrier. You could tell me what the best gun is or the best game to go on like a team deathmatch whatever but please could you tell me something! Thanks. Video games....