Is your grandfather like Crisi 's:
"... my late grandfather who taught me how to play when I was 6 years old... and never let me win, I was 12 before that happened!"
It's too late to cram. Play your normal game - but try to maximise you psychological advantage. You're probably playing in his house. You're 13 years old. Maybe he's got some valuable items sitting around that you could pick up and play with during the game. Talk about football, TV, games consoles and winter fuel allowances. Get him to reminisce about the old days; then just before the end of one of his stories, change the subject completely without warning. Mention other grandparents who "lighten up" on their grandchildren.
My grandpa is really good at chess, I'm only 13 and recently started.
I'm going to be playing him in a few hours.
Anything I should learn beforehand?
Heres the last game I played today:
Chessmatch1