I am terrible at fortnite because I cant build. What should I do?
Play the respawn large team modes. It's less frustrating to learn while playing those since you usually get materials faster per tree/fence/whatever and you don't do collecting for nothing if/when you die at your first enemy in 5 seconds.
Learn to collect materials (hit the blue reticle that appears!) Don't worry too much about the kind of materials, just learn what are good, quick sources & locations of different materials and just try to have a combined number of at least 500-600 materials at all times (I try to have at least 500-600 wood + 200-300 bricks and metals each).
Trees, trucks and bigger rock formations give you a large(r) amount of materials per destructed thing but there are also some smaller things like fences that, while they might only give 10-15ish mats per piece, they are so quick to destroy that they accumulate pretty quickly. And then there are things (especially metal ones) you absolutely shouldn't waste your time on unless you are desperate for some mats (i.e. early game and you landed at a place you saw other players land to as well, it might be worth it to wreck pretty much everything within a house, just to have as many materials as possible to at least be able to put up a few walls and to get to the top of a building).
Once you've got mats, at first just focus on basics like getting a wall up quickly to the direction you are getting shot at from, and to build stairs quickly to get over steep slopes/high obstacles/on top of a house. Once those go well, learn to do things like a quick shelter box (walls in at least three directions and stairs within them to let you peek around a bit) and you can start maybe trying to push players and get into build battles.
When it comes to build battles, getting the upper hand and blocking the other player getting a higher ground is key. Easier to aim downwards than running with your aim up high. So if someone is rushing you, be aggressive rather than just backing away and being on the defensive. Once you've got higher ground, be patient. You're in the more advantageous position, don't give that up but peek & shoot strategically and block any tries from the enemy trying to get past you (good headsets help with this, you need to be able to hear where your enemy is).
And while getting to high ground, don't just rush forward while spamming stairs only. You'll be really easy to shoot down. At the very least do stairs -> wall (underneath the piece of stairs you just built) + maybe a floor piece as well if you've got mats to spare. Repeat that until you're up to the other player. It won't help you a whole lot but it can give you just those extra 2-3 seconds to connect to the other player's structures vs only having a single stair piece that can be destroyed easily & falling down and being at a disadvantage.
Don't build a full cube just to turtle up in it. It's ok for a quick, brief pause to gather your thoughts in intense moments, but good players will know how to fuck you up within one if you try to set up shop in one, so you're just delaying the inevitable and not learning to properly battle against good builders. If you find yourself accidentally in one, learn how to get out of one quick (if it's enemy's pieces or wood or other materials that were just placed & still building, you can just hit them once or twice, if they've been there longer or your own pieces, edit a hole). If an enemy pushes him-/herself into one, it might be time for a quick, well placed spike trap. Or if you know exactly where the enemy is coming from, it might be a good idea to line your shot, wait until they break your wall and then blast them to oblivion with a shotgun the nanosecond the wall breaks.
Once you got the hang of building basic stuff, start to learn editing those parts. Learn how to quickly make small holes into floors that you can shoot through or bigger holes you can move through. It can be quicker to break walls by just hitting them with the pickaxe but that makes a loud sound, which you might not always want, so editing a bigger hole to run through can occasionally be preferable..
Ultimately there is no other way to learn but by just learning by doing. You will suck at first but if you keep at it, you'll become better.
Ps. Make sure you have builder pro mode in your control settings. It ties different parts to different buttons instead of having to cycle through parts. Much more intuitive and faster.