I don't know how to beat my grandpa, he's really good and I don't know if he's beatable. He's hardcore when it comes to chess.
I'm a chess coach and a chess master, so I'll tell you what I tell my beginner level students.
What worked best for me to improve my middlegame when i was new to chess was solving puzzles in the book 303 tricky tactics by Fred Wilson and Bruce Albertson to get tactically strong, and to read the book The Amateur's Mind by Jeremy Silman to get positionally strong. Rereading either book more than once will only make you better.
If your grandpa is too good you might also need to read Silman's Complete Endgame Course by Jeremy Silman. This will give you a basic understanding of endgame positions that should be enough to play up to the 1700-1800 level in endgames.
As for openings it's best to bring your pieces to the center, and try to get all your pieces out before you move any of them more than once. I'd suggest searching the italian opening and the colle system on youtube, and check out some free videos masters have made on them. they are good openings for beginners and get you a reasonable position to try and play from.
Goodluck beating grandpa!
I can't figure out how to just comment so here's my answer:
I've been studying chess for a couple of months now and I started with Bobby Fischer Teaches Chess. It's nothing profound but it'll help you get a basic understanding of mating and how to mate from different positions. From there I read Pandolfini's Ultimate Guide to Chess. It wasn't the most interesting book but it offers a lot of insight into the motive and consequences behind various moves. I then moved onto Logical Chess: Move by Move, which contains a number of world champion chess games but the author thoroughly explains every single move.
Studying these texts has been really helpful for me, but in order to develop the skill you'll read about you need to supplement it with your own playing and application.
Becoming good at chess is no easy feat though. Depending on how good your grandpa really is it may take quite a bit of studying to beat him.