You have a few options. You can join a college or university course that teaches Modern Standard Arabic, although it may cost you a few hundred or thousand dollars. If you want to immerse yourself in the language, you can travel to an Arabic speaking country and take a course in the local dialect. Traditional madrasahs will teach you Classical Arabic, but you may need to commit to the full four or five year full time course. To focus on Classical Arabic, you can join a well established and reputable online course such as the Shariah Program and learn Arabic online.