Device:

If possible, use a good computer. If you are experiencing technical problems during your lessons, try switching to another device. If your computer is old, your phone or tablet might work better.

WiFi:

If possible, use a wired connection from your computer to your router. If you use WiFi, please make sure you can see your router from where you are. Even better: Have your classes in the same room as your WiFi router. The router provided by your ISP might be a cheap model, so please consider upgrading to a better router and having it professionally installed.

Internet Provider:

Your ISP won't advertise this, but with most internet plans you're actually sharing your bandwidth with your neighbors. For example, cable internet in my part of Latin America is shared at a 8:1 rate. This, compounded with the fact that cable internet usually has low upload speeds (a paltry 1,000 Kbps in my country) means you can end up with an upload speed of just 125Kbps despite your plan being advertised as "10 Megabits". And that's below the bare minimum for Skype video calls. With cable internet, even "50 Megabits" plans have a poor 1,500 Kbps upload speed, which can become only 187 Kbps when your neighbors are video chatting with their family in New York.

If possible, upgrade to a plan with a lower sharing rate and a higher upload speed. Check if your area has fiber available.

Sound:

Please use a headset. Even a cheap headset is better than just using the integrated mic and speakers in your computer. You might hear your partner/teacher/student very well without a headset, but he/she might have to put up with a lot of echo/feedback on his/her side. Do it for him/her.

Video call service:

Have a backup ready in case your preferred video chat service goes belly up. Currently, the best ones I've used are Google Hangouts and Zoom. Zoom is free for calls between two people, no matter how long the call is. Google Hangouts is dead simple to use and there's no need to download anything.