The best way to learn Japanese is to be taught by a professional teacher, but if that's not an option then my suggestion is getting the Genki textbooks, their accompanying workbooks, and a book for studying Kanji.

Once you have the basic alphabets of Hiragana and Katakana out of the way, just follow along with the Genki book. I'd suggest spending about a week and a half on every chapter, just to make sure that you understand the concepts.

One thing that would be helpful would be to find a language partner on iTalki or somewhere else, who can help confirm that you're getting everything right. 

Also, there's a very useful app called Memrise, where people make courses for a bunch of different languages. Some people have uploaded the vocabulary and Kanji from the Genki textbooks, and you can test yourself on them with Memrise to make sure you remember everything.

The most important thing to do is practice! Immerse yourself in the language; watch anime, listen the Japanese music, watch Japanese language films, and practice what you know for at least a half hour every day. Good luck!

がんばってください!

At Duolingo, choose "I speak Japanese" then take the "English Course for Japanese Speakers". Learn backwards and use google translate to interpret kanji. 

Use http://www.guidetojapanese.org/learn/ for grammar help, 

http://www.nhk.or.jp/lesson/ for basic language lessons, http://www.omniglot.com/language/phrases/japanese.php for helpful phrases and links to other sites, 

http://www.japanese-online.com/ for review and fast-paced learning,

http://ocw.mit.edu/courses/global-studies-and-languages/21f-501-japanese-i-fall-2012/online-resources/lesson-1/ for worksheets,

http://web-japan.org/kidsweb/language for kid/beginner friendly learning,

and https://www.merlot.org/merlot/materials.htm?category=2481&hasCollections=false&isContentBuilder=false&filterOtherOpen=false&hasAssignments=false&hasAwards=false&filterSubjectsOpen=true&hasRatings=false&filterTypesOpen=false&filterMobileOpen=false&hasComments=false&hasCourses=false&isLeadershipLibrary=false&filterPartnerAffiliationsOpen=true&hasSercActivitySheets=false&hasPeerReviews=false&hasEditorReviews=false&; for a directory of helpful sites

Hey, I found that for learning basic Hiragana Dr. Moku's Hiragana Mnemonics and Memrise really worked for me. Also, the The Third Ear Rythmic Japanese (Music) and Earworm's Rapid Japanese (music) are a lot of fun. Hope this helps you! ????