I would seriously suggest a class or at least some tutor with a good book to learn the fundamentals. It is very difficult to iron out things you learned wrong. I met a few people who speak a horrible Japanese, because they mostly picked it up (very bad pronunciation which makes even mine look native, almost no particles, very little grammar and prone to even using that wrongly, but know a lot of very casual terms or manga vocabulary). If for nothing else, you will want someone to correct your mistakes (especially pronunciation). Japanese people will neither correct you nor will they understand you if you pronounce wrongly. Things like a long or short vowel which are very similar for a foreigner are completely unrelated words for Japanese. Teachers may learn the trick of guessing what you tried to say, but most others won't.
If you have some fundamentals, emerging yourself in as much Japanese as possible is a vey good idea to pick up more vocabulary and phrases.