I understand from your query that you are a teacher for some subject, other than English, but you are keen to speak the English Language fluently. There is absolutely no need to worry. You have the innate desire to get comfortable with the English Language, which by itself, is a very positive sign. Down here, where I teach the English Language, lot of teachers from various disciplines, and people with varied profiles have approached me expressing their desire, to learn the English Language.

There is only one method. Try to identify one person who speaks the English Language fluently. You need a guide or mentor to take you through the initial learning process. Ensure that you have a talk with this person everyday. It can be a casual discussion. Ask him to keep on correcting your mistakes, which will ensure that you gradually get the confidence part going for you. Initially you need not focus on grammar. The way you picked up your mother tongue, is precisely the same way, you need to follow, to get comfortable with the English Language.

Let me tell you that this process is very demanding and you need to put your heart and soul into the learning process. Simultaneously, you can start seeing the BBC news channel or National Geographic . Also try to read magazines related to your area  of passion or interest.

Good things take time to happen.

All the best.