As you probably know, WiFi only works if you have a functional router. But you can't just put a router anywhere and expect peak performance.
Is your router in your basement, or in a cupboard, or generally far away from the devices you use? If so, consider fixing that. Your router needs to be in as central a location as possible.
Even if you can't, you can still improve your signal by putting the router in an elevated position and making sure there's open-air between it and any device you're using. Don't put it on the floor if you can help it. Walls, especially thick ones, can also be hell for wireless signals. It's possible for other electronic devices in close proximity to the router to disrupt the signal, too.
In general, keep the router away from other devices, don't make the signal pass through too many walls, and keep it up as high as you can. Of course, you could always try limiting the number of devices you have connected to WiFi, too.
On the cheap end, you can get some ethernet cables and try to wire up as many of your devices as you can. I realize this is inconvenient and will invariably lead to cable clutter. Still, if you're having signal problems, a direct connection between your device and your router should help stabilize things.
For those at wit's end after trying the above, you might simply need a new router.
If you wind up getting a new router, make sure it has a good number of ethernet ports. You don't want to be stuck shuffling wires in and out of a limited number of ports.
Maybe you have a decent router, but it doesn't cover your entire home on its own. This is where mesh WiFi systems or WiFi extenders come into play. These are neat little gadgets that can lock onto your router's WiFi signal and belch out a weaker, but normally usable version of it in a different part of the home.
If your signal strength is fine but speed is an issue, one way to go is simply paying for better internet. We hope nobody is forced to do this right now, but the option is there. You'll need to consult your ISP to find out what speeds you're currently paying for and if it's possible to get a speed boost.