Do I need to learn Thai?
Yes and no! It will depend, to a certain extent, on your lifestyle here but bear in mind that most taxi drivers do not speak English. It is certainly worthwhile learning some basic Thai to make communication with taxi drivers and shopping a little easier. Some basic Thai does not take long to pick up and most people find it enough to get by. Keep in mind that Thai is a tonal language and even though you think you are pronouncing the name of your street correctly, if it is not the right tone, your taxi driver may not understand!
The Bangkok Guide lists all the available options, from Thai language schools to private instructors, for learning Thai.