guys,i have a 25' inch Hull.how to get a CoG?

You should just be able to pull it out of the air.
LOL what type of boat, who makes the boat, what is your setup.

i make my own Hull,its a Deep V.

What I have heard is the CoG should be about one third (33%) of the hulls length. measured from the transom.
So using this as a guide a 33% of a 25" hull is 8.25".
This is a good starting point and should be checked with all of the hardware -
motor,esc,batteries-etc in place.
Hope this helps.

Edit' Iv'e been reading that 30-33% is the range that most folks are using to find their CoG. Mabye start with 30% and work from there.