Instruct the majority, if not all, of your class in English. Your students may come from various language backgrounds that you are not familiar with, so using English as the main language of the class keeps everyone on an equal playing field and also gives them an opportunity to hear you speaking fluently.
Though grammar can be challenging or uninteresting for some students, it is a crucial element in learning English (or any language) properly. Introduce one grammar concept at a time, give hand-outs and explanations about the theory and then offer a variety of exercises to practice these new concepts (group exercises, spoken exercises, written exercises and so on).
Some students who have been learning English for years never feel completely comfortable speaking the language because they don't have enough chances to practice while they are learning. To help your students speak and understand the language well, implement speaking, listening and reading exercises into your lesson plan. For example, students can partner up and ask questions and give answers in English. You can also assign a topic for them to read about and ask them to prepare an oral presentation for the class.
Be consistent in your methods. For example, if you have a two-hour class, spend the first 15-30 minutes correcting homework from the previous class together. Spend the rest of the first hour reviewing concepts from the week before and how they will lead into the next concepts. The first part of the second hour can be spent briefly explaining a new theory and giving a handout containing this information. The rest of the class can be dedicated to practicing this new concept through group activities, written activities, movie clips, audio clips or whatever you find interesting. The content of each class will be different, yet the basic structure gives students an idea of what to expect and helps them feel comfortable learning with you.
During class, if a new word comes up from the homework, an in-class reading or from a student, use the opportunity to explain its meaning and other words and meanings associated with it. Try to avoid just giving bland definitions. Teaching some colloquialisms and other interesting bits of information about anglophone cultures is also useful for students learning the English language.