This may come as a shock, but things will happen in your classes that you don't expect! Students will finish some activities too soon and take longer on others. Some things will be easy for them, and others that you expect them to understand quickly will be tougher. It is more important to be flexible than stick rigidly to something that isn't working.
2. You don't need to know all the answers.
Another shock? I taught some classes where they asked me questions I couldn't answer right then. I told them I would find out for them, and I did. As long as you are honest, say you don't know, and then give the answers later, that's fine.
3. Every student is different.
Now, that may be obvious, but so many teachers forget this when they are teaching and treat each student in the same way. That won't work. Get to know your students, their learning styles and their strengths and weaknesses. If possible, create a variety of tasks to suit different students in the class.
4. You can't force students to learn, nor should you.
Teachers often feel responsible if their students don't progress. You can give them the tools for learning and inspire them to want to learn, but then it becomes the individual student's responsibility to learn, not only yours.
5. Have a personality.
Be yourself. Let the students see you're a human being and not a teaching machine. Laugh at yourself if you make mistakes. Bring humor into the classroom.
6. Be encouraging.
Error correction is essential in the right circumstances, but if you correct every mistake then you can easily destroy a student's confidence. Praise good work and never tell students they are stupid. Students who think they are stupid lose motivation.
7. Balance your lessons between different skills.
A teacher who teaches 90% grammar is not helping the students. Communication skills, reading, writing, and vocabulary building are all as important as grammar. Possibly even more important than grammar!
8. Keep an emergency resources box.
Include games, paper, colored pencils, extra worksheets, magazines etc. This will be invaluable if you need to add a new activity into the lesson or to extend something the students are working on.
9. Give homework.
This is controversial! You can't force students to do homework, but if you give them the choice then they will feel encouraged and they can take responsibility for their learning. It is really important to mark the homework if you do give it, and not leave it on your desk for weeks.
10. Have fun!
If you enjoy your lessons, then your students will too.
These are the pieces of advice that helped me learn how to teach English and I'm sure they will help you too!