How i can make my own asset for game engine ?

Question

i want to make a game

Answer 1

What type of asset? Asset is a broad word

Answer 2

what is best game engine in which we can get many free assets.

Answer 3

'best' game engine is subjective. I used to work with Unity a lot, since it has a Huge community. Furthermore, you can make 2d assets in a paint program
3d assets are made in 3d-editors(like Blender).
Free assets can be found anywhere, because assets are (mostly) universal.