The Internet is not optional: Whether we're working or streaming TV, we need an Internet connection that just works. Some issues are the fault of your Internet service provider or your equipment, in which case there's only so much you can troubleshoot without calling tech support. But we've assembled a checklist you can go through when your Internet stops playing nicely. To keep the list a reasonable size, we're assuming that you've used this Internet connection successfully before, though not necessarily with the device that's giving you trouble.
Homes are full of things that can mess with Wi-Fi: a microwave, a thick wall, a neighbor's network. Microwaves operate on the same 2.4GHz frequency as most Wi-Fi connections, so don't put your router in the kitchen. However, if you have a higher-end router that's broadcasting at 5GHz, use that instead and avoid congestion, assuming that the device you're connecting to it can also operate its Wi-Fi connection at 5GHz. These dual-band devices cost a little more than the vanilla variety, but the expense is worth it if you live in an apartment building. Some mobile devices can even bounce between the two networks, depending on which one is providing the strongest signal at that moment.
You may have a modem from your cable company that comes with its own router. Those are bundled with the service by default. They're generally not as good as modems and routers that you can buy from the store. Since the cable company charges around $10 a month to rent the modem to you, buying your own equipment will eventually pay for itself -- but your service provider may only provide tech support for the gear that it rents. Also, your service provider usually will give you a replacement unit at no additional cost, if yours is faulty or damaged. So there are trade-offs between having a fast and potentially more advanced device of your own, versus renting one from your Internet service provider (ISP) that will help if your router experiences issues. Also, your ISP won't normally set up your service with the hardware that you purchase. Instead, the ISP will charge you a moderate fee to set up its own gear (unless there's a promotion for free installation).
Boosting your Wi-Fi signal
The higher the Wi-Fi frequency, the more easily it's blocked by walls, so there are trade-offs. Get around blockages by adding a range extender in the hallway or a power-line adapter in the room that doesn't have the router. Range extenders are basically miniature radio towers that your router and Wi-Fi-enabled device can bounce off. It's like putting a mirror in the window to bounce sunlight into a spot where the light wouldn't otherwise directly hit.
Power-line adapters plug into a nearby electrical outlet and use your home's wiring to send data back and forth. This is possible because electrons can be used to send both power and data. So you use this adapter wirelessly on the end where your devices are, and the router uses another adapter on its end. The latter adapter connects to your router with a standard Ethernet cable.
Both range extenders and power-line adapters require power from an electrical socket, so make sure that you have one near where you want to put the device. Some adapters have Ethernet ports as well as Wi-Fi radios, so you can physically plug in a nearby desktop or laptop computer that has an Ethernet jack -- basically using the power-line method as a really long extension cable.
If your laptop has no Ethernet jack but has a USB port, you can buy a USB-to-Ethernet dongle to connect your cable anyway. Some dongles come with their own USB ports, allowing you to keep using the laptop's own port for additional USB devices, like mice or external speakers.
Troubleshooting unexpected slowness
Slow downloads and stuttering streams can happen even when you have good Wi-Fi reception and no apparent issues on your end. The reasons vary. Sometimes the website or service that you're connected to is having problems. You can test this by just downloading or streaming from another website or service and comparing the quality of your result. Other times, your Internet provider is having a technical problem, or too many other nearby users of their service are downloading a lot of stuff at once, creating congestion for that section of the neighborhood.
If you check your connection on speedtest.net and get an unusually slow result, the problem may be on your ISP's end. Your ISP may have a service status page that you can check for issues, or you may have to call its tech support number to figure out what's going on. If you using a game console, you can find Microsoft's Xbox Live status here, and Sony's PlayStation Network status page is here.
Lastly, keep in mind the difference between megabytes (MBs) and megabits (Mbs). Internet connections are advertised in megabits because, well, this number is eight times the size, so it sounds fast. That means that your 20Mb service is actually a 2.5MB service. And you won't get those speeds in practice. You'll get "up to" 20Mb. There's always some congestion on the network, and some data needs to be requested again and sent a second time, in case it didn't go through successfully the first time.
This means that you may pay for 20Mb but download at 2MB per second. Netflix recommends 5Mb per second for each HD stream, so if a family of four is streaming HD to four different devices at once, you may get stuttering even if your connection is officially rated for 20Mb. Sometimes you can have a technician come out and test your home's coaxial cable wiring, but that service usually has a fee attached. The tech may be able to boost your connection (or in some cases they may make it worse).