How to use external flash with nikon d5300?

I've just bought nikon d5300 from bangkok, i don't know how external flash works because even after attaching it with camera its built in flash pops out

If your camera flash is trying to deploy when there's a speedlite attached to the hotshot then there's a fault with the camera.

It might be your flash. Is it recognized by the camera? If it the camera does not know it's attached, it would pop-up as you describe.

Or it could be the camera. Did you buy it from a reputable and authorized Nikon dealer?

A combination of two things going on.
If you have your mode set to Auto, then when the light is low it want to choose flash.
If you have a flash mounted on your hotshoe, and the built in flash pops up, then it doesn't know that the flash is there.
If it is a Nikon brand TTL-compatible flash, the camera should recognize the external flash unit, when it is turned on. If it is turned off, then the camera does not know that it is there, and so it wants to pop its built-in flash up. So don't touch the shutter button until your flash is on.
If it is an off-brand flash there may not be a TTL electronic interaction. So the camera won't know its there and may flip the built in flash up. It might trigger the external flash anyway, but exposure is very often going to be wrong.
Some of these confusions will go away if you shoot in P, A, S, or M mode instead of Auto.
I usually use A, sometimes M. My camera is a Nikon D90 and flash is Nikon SB600.