I'm making a game called Sword Game and I need help, anyone?

So the basic idea of this game is a sworded version of the Call of Duty gun game. I know how and what the scripts should do but I just cant find out how to write them.

Examples:

1 kill with windshear = firebrand

1 kill with firebrand = Venomshank

1 kill with venomshank = illumina

1 kill with illumina = darkhearts

So they start out with the default sword and as they get kills they get new swords, every 1 kill grants the next tier of the sword. If you still don't get what the game is suppose to be about feel free to leave a question asking so.

Use int values, so when someone kills someone with the wind shear do player. Wind shear kills.value +1 or something, and make a gui to load the next gun. Pretty simple really.