How to get better at fortnite

The only way to get better at something is to practice,
If you can just play game after game after game, learning from your mistakes, you'll be a really great player in no time.
There's different mechanics to focus on, such as Item loadout and sorting, so you can quickly switch between things using keys instead of scrolling. This can save you time and often your life in games like Fortnite.
If you suck at building, just practice building towers, little launch pad rooms and little bases for when you're getting shot at. Switching and building the doors on things sometimes gets people confused, but if you're able to throw together a building in the middle of a gunfight, it can often save you and/or your team..

The only thing you can do is play, find out why you failed and play again, learning from your mistakes. If you play with people who are better than you, they'll usually be more than willing to lend a helping hand and help tutor you.