You should do some research. The common misconception is to just fit an adhesive backed mat, like dynamat etc.
There are different types of material, all achieve different things. Dynamat type stuff works poorly to absorb, or block sound. Its primarily for stopping a panel from resonating.
A foam / vinyl pad will absorb more, but deaden less etc.
So its important to decide what you want, and go from there.
As an example, high end cars dont cover the insides with deadening pads. They have some on large panels (they dont need to cover a panel, just a certain surface area of it to be effective), they mostly have thick foam, backed with vinyl. Thats what makes them quiet.