I don't know how to fill in the house sale stuff. What is the cost base at disposition?

How am I supposed to figure all that out when we lived there for over 30 years?

Investors and rental owners

The term "cost base at disposition" basically means what was the price of the house when you first bought it 30+ years ago. If this was is your primary residence for all these years - you should not pay income tax on the proceeds of this sale.