I am making a game in unity it a 3d game but i nedd 3d model and i do not want to download any 

please how do i make 3d model

Use 3D modelling software.

Blender is what a lot of people use (including myself).

There's also Maverick Model 3D. The site is down as of this writing, but it's a very nice, lightweight app. Much better for beginners, and widely ported.

if you need low poly "asset forge" here in itch.io is a good 3d software for beginners