Decide why you want to learn Arabic and which dialect you want to learn. If want to learn Arabic primarily for prayer or reading the Quran, focus on classical Arabic. If you want to learn Arabic for travel or business, it's best to start with Modern Standard Arabic (MSA) to get grounded in the basics first. If you simply want to learn a few phrases for an upcoming trip, then you're better off skipping the alphabet. Learn conversational phrases instead. Keep in mind that each Arabic-speaking country has its own dialect, so evening greetings and words for common foods can vary from country to country.
Learn the Arabic alphabet. Some online sites offer audio aids to help you learn the letters. The downside of learning Arabic script online is that you can't actually practice the writing, and exercises are focused on listening and comprehension rather than on production. If you feel limited with online options, buy an inexpensive workbook like Teach Yourself Arabic Script to help you learn the correct pen strokes and patterns for each letter. Combine this with the audio materials online for a more comprehensive learning experience.
Learn basic conversation and grammar. At this point, you should choose the dialect you want to learn. For Quranic reading and prayer, consult an Islamic site like Shariah Program. You can learn basic conversational phrases from Transparent Language. All resource links are listed below.
Practice conversation or prayer with a native speaker. Sign up for My Language Exchange to connect with native speakers for free. Alternatively, purchase a program like Rosetta Stone Arabic Totale that gives 12-month online access to course materials, games, chat rooms and live tutor support. Purchasing Totale instead of the traditional CD-ROM means you can access your Arabic course from any location.