There may be several language institutes in Doha offering courses on improving English. Berlitz Language Center is one of them. These institutes are usually based on classes you need to attend and they may not be able to give you the individual attention you may need apart from the high fee they will be charging you. I suggest you get hold of a private tutor who could give you a one-to-one attention. In this way, you will learn faster, and it will also be soft on your pocket. You do not need to go for a native English speaker as most of them I have come across so far, have poor knowledge of English.