when putting sod down do i need to kill the old grass or can i dig it up?

You can just 'lay' the sod over your old grass, but it can be a 'strong grass' that could grow through the new sod. You would be 'best off' if you dug all of the old grass and laid a proper 'base' where the sod will grow ... and that 'base' depends on what the 'soil content' is where you live. Go to a good 'nursery' near you, and take a sample of the 'soil' and ASK ... and BE SURE that the 'sod' you are laying is 'right' for your area.