This is one way to talk about your grandfather.

When talking to someone in your family, you call your grandfather:

Grandpa

Granddaddy

Granddad

Each family has their own way of talking about grandparents, but these are some of the most common ones.

When you're talking about your grandfather to people who aren't in your family, you include "my" at the beginning:

my grandpa

my grandfather

Some of the less-common names for grandfathers sound silly when you're saying them to people outside of your family, so people mostly just use the two choices above. Also notice the difference in capitalization.