You need to be almost perfect at the language you want to teach. Specify if the level you are teaching is native, because your student sometimes cannot tell the difference. Note that your student may not even know the alphabet pronunciation of your language. Be prepared for a very basic lesson. A good teacher is patient and can repeat a simple instruction several times. Be prepared for very absent minded students. Never lose your temper. In my opinion, you must not try to be a teacher if you are impatient and/or you have stress problems.
You also need a good microphone. You should buy quality headphones with a long cord to be able to sit properly and talk to your student. This is very important to avoid low back pain problems due to a poor posture. Do not underestimate this pain. You can also buy wireless headphones with Wi-Fi or Bluetooth. If you are using a desktop computer, you might need a dongle to be able to connect your computer with the device. The dongle should not be expensive.
I teach for free therefore I can select my students. They are not my clients. There are two major types of students: the talkers and the listeners. I am a listener and I can only teach to talkers. If I try to teach to another listener, there is a big boring silence. Whereas talkers feel very good when someone can listen to them and correct their pronunciation or simply attempt to participate to the conversation.
The key is to continue teaching a student if there is an interesting conversation going on. What is interesting? It depends on your tastes. It is not necessarily linked to common interest. It is just a feeling. You must also like the voice of your students. Be comfortable with their ideas. Ensure that they also give you a good sound quality. Are they respecting you? If all these simple conditions are not met, you should not teach to this person. Try another student who needs you just as much, if not more.
To get potential students, try very popular language exchange websites like Mylanguageexchange. To be able to find students, you generally need to be a paying member. People will not notice your profile unless you email them first through the website. That is if you are a man. Women often get emailed without becoming a full member. In your email, you should specify your native language(s), your interests, and your intentions to teach for free. I also say that I want to become friends and I give my skype username (avicoco).
You need to send as many emails as possible, only one per person. Never insist if a person does not reply. Out of 100 emails you will get only one interesting answer from a student that you will enjoy talking with. Try to target your profile search to the students you want and send just one email per day. If people want to learn a language, they will contact you. Once we are on skype text chat, I always ask directly if the person wants to audio chat (not video). The first time I met her, my best friend on skype almost immediately replied: "Yes Ok!”. No convincing required. Do not waste your time on those who need convincing, as there are people who want exactly what you want right now. Look for them. It’s a win-win situation.
I speak to this person almost every day, and it is very fun and casual. And the student made excellent progress, because one learns naturally through interesting conversations. In exchange for my knowledge, my student friends give me information on their part of the world. I learn a lot thanks to this, more than with a thousand books. Also theirs is not the media point of view, rather real people's ideas and comments and that is much better. Sometimes you can talk to people in war zones and they give much more accurate news than the TV. You learn about their culture and their jokes. And I am also their student, if they want to teach me some skills. It can be anything. They like to talk about their job and their dreams.