How do I get a knife?

If you are playing on Android just go to the customize hud, there is an option to use the knife. Melee kills can be performed automatically when you are really closed to an opponent and press that knife.. Just one suggestion to perform this, put that knife icon on shoot icon by customizing the hud so that even if you are shooting and you get really close to the opponent it just performs the melee kill itself.

If you are playing on a PC - it's the letter 'E' on the keyboard to use the knife for a Melee Kill