How do I make a new weapon?

This is a very vague question, ie, not understanding your level of understanding on how to make new weapons in the game (modify the string files, entity files if you want to create a new module or shiptype or just new abilities and linking that from existing ships etc.)

If you are looking for a AOE damage type then I would look into the "AbilitySelfDestruct" and follow that through to the "BuffSelfDestructSelf" & "BuffSelfDestructTarget" to determine how damage to multiple enemies within a radius can be accomplished.

Then you would have to create a new ability on a certain type of ship that has a cooldown every 1 second or so.

Just a thought on how I would go about and accomplish it.  It's almost like adding a smartbomb ability on a ship...

People are much more likely to respond if you try it on your own and start linking some of the text of areas you have modified, ie...

 targetFilter
  numOwnerships 2
  ownership "Friendly"
  ownership "Enemy"
  numObjects 4
  object "CapitalShip"
  object "Frigate"
  object "PlanetModule"
  object "StarBase"
  numSpaces 1
  space "Normal"
  numConstraints 0
 range
  Level:0 8000.000000
  Level:1 12000.000000
  Level:2 0.000000

and

instantAction
 buffInstantActionType "DoDamage"
 instantActionTriggerType "OnDelay"
 delayTime 0.000000
 damage
  Level:0 4000.000000
  Level:1 8000.000000
  Level:2 0.000000
 damageAffectType "AFFECTS_SHIELDS_AND_HULL"
 damageType "ENERGY"
 isDamageShared TRUE

don't forget that you have to make it a missle or projectile to make it lauch because if you beam, well the results are obvious