people learn Arabic for a variety of reasons: for work, for travel, for religious purposes, because of marriage or friendship with an Arab, or simply as a hobby. The motivation to some extent determines the most appropriate learning method.
The first thing to decide is whether you want to learn standard/classical Arabic or a colloquial dialect.
Unless your interest is confined to one particular country, the safest option is to learn a version of the classical language known as Modern Standard Arabic. This is what is used in books, newspapers, radio and television news programmes, political speeches, etc.
Using standard Arabic in everyday conversation sounds a bit formal to Arab ears, but at least you can be sure of being understood by educated Arabs anywhere in the Middle East.,and it is relatively easy to adapt to a local dialect later.
Among the dialects, Egyptian and Levantine (spoken by Lebanese, Syrians, Jordanians and Palestinians) are the most widely understood outside their specific area.
If you are planning to learn Arabic because of an interest in Islam, standard Arabic is preferable to a colloquial dialect.