Hi
 
I'm using my digital piano to accompanying my vocal when I'm sing.
What the best way to recording a track is it by Separated track? So the track one is the chord only?
And the second track my voice?
 

Yes. If you want to edit vocals and accompaniment separately, they need to be on separate tracks.

Make sure you listen (monitor) your piano through headphones so that the piano sound doesn't bleed into your vocal track

Thanks. Can you guys help me doing video tutorial about it and about the next step of mixing. I'm start from 0 here

You need to make sure the 2 tracks are routed separately. click on the "Track input" drop down menu. 
How are you getting voice and keys to Sonar ?

I use My Yamaha mixer Mg10xu
Plug into it my keyboard and my mic. I know How I record Now in sonar. But what the step of mixing what after recording.
Best regards.

Might be worth the $15 investment for you.
Groove 3 SONAR Mixing Tutorial

Good Day,
Id suggest by coincidence you refer to the most recent "Friday's tip on the week #163" at top of this forum, and also refer in Sonar help menu sub-section on mixing to get started.
Also, although somewhat geared to their products there are generic principles in Izotope's Mixing Guidlines (free pdf)
https://www.izotope.com/en/support/support-resources/guides/mixing-with-izotope.html
 
 
Cheers 

A 1 2 3 4
 
 
Sorry.