how i can start a main bus?

[quaint timber]

Mainbus is just training wheels of design, but just build straight lines and use splitters when you want to build stuff

Putting wires on the mainbus is generally not good since it takes more space than the copper plates

[pine berry]

If you want to go the main bus route, you want to have your production facilities perpendicular to the bus.

So in your base, if you have your smelting at the top, and the belts running down to the south, you want all your production facilities to be aligned horizontally to the left/right of the bus.
Right now you have them all vertical in the location that the bus is supposed to go.