Studying Japanese can be a long and challenging journey, however I can assure you that it is a journey that will bring priceless memories. No matter your age, anyone can learn Japanese, so never assume you cannot make it or if you believe you are too old and your time has passed. And always remember, don't let your dreams be dreams.
I have used many entry level Japanese language books over the years, and I've noticed that the best books to get started with the Japnaese language is either Genki or Japanese For Busy People. The Genki book is an all-around book meant for college level students, and the material in the book is tailored to those seeking a comprehensive approach to Japanese. Japanese for Busy People is another great book with a more sped up curriculum, and it is a great resource for those planning on working at a Japanese business.
When I started learning Japanese, the very first thing I did was memorize the Katakana characters by heart. Flash cards are your best ally in helping you master Japanese, and with the advent of technology, many options exist to help you memorize various content. For those wanting to use their computer to study, Anki is a magnificent open-source application to help you memorize pretty much anything! Also, always talk out loud when using flash cards, so when you see a character or word and recognize it, like "オ", say out loud, "O".
Writing Japanese is very very important! With repitition, come mastery, so practice writing as much as you can. When learning new vocabulary, personally I like to write the word first, then say the word in Japanese. I usually do this with each word about 20~30 times each. Then when I feel confident, I practice using the flash cards. Now which ever words you've forgotten, it's ok, write the words again and repeat the procedure until you feel confident.
After you awhile, once you've gotten a lot of verbs, nouns, and adjectives remembered from a chapter, try to come up with your own original sentences. Play around a bit, and have fun with it. Then once you feel ready, hop onto the next chapter of whichever book you are using, and repeat the entire process again.
When you're at the end of either book, you should try to definitely communicate with other people. If you are feeling brave, please also try communicating with others while going through the book. Some places to try to meet others are on Discord, HelloTalk, Twitter, Instagram, Forums (I use ResetEra or NeoGAF), or meet a group locally in your city/town with meetup.com
I wish you good luck, and never give up!