While online courses as suggested by other QLers may be helpful in giving you a start, the best way to learn any language is by mingling with the locals. You then get to pick up the phonetics and it's a faster mode of learning. I am not an Arabic speaker but I learnt the language by conversing with the Arabs.
It may be a bit difficult to learn the phonetics if you are from the west as Arabic has many words where the sound originates from deep down in the throat. So you need to keep plenty of throat lozenges handy should go on to take the Arabic language course!