I am thinking of moving to Japan in the future, and I know that I will have to pass JLPT exams in order to get a job. So how should I start learning?
The best option is to get lessons, either in a class or from a private tutor. It would be a good investment since a skilled teacher will be able to guide you through the early stages in the learning process.
If you cannot take a class or private lessons, I recommend buying a good textbook for self-study. The 'Genki' series is considered a good option for self-study because it covers a variety of grammar structures and has supplementary workbooks to test your progress. In general, beginners learn the 'kana' scripts called Hiragana and Katakana first. These allow you to read and write Japanese phonetically while you are learning grammar and syntax. Focus on learning pronunciation properly by referring to sites such as Forvo. From there, you can begin to start learning 'kanji', the more complex script. Your study should include a range of audio-visual materials, reading materials, quizzes and tests, and your coursework from the textbook. If possible, you should practice your speaking with a Japanese person. I also recommend using spaced repetition programs (such as Anki) to review the material you have learned.
I suggest postponing the JLPT study until you have first completed the basics above. When you are more comfortable with Japanese, there are many sites and resources which are dedicated to the JLPT specifically.