Genki is a Japanese textbook. The set comes with a textbook and a workbook to fill out. There are two levels Genki I and Genki II, I would suggest only buying the first one for now so if you end up not liking it you're not stuck with 4 books that you don't want. Both textbooks also come with a CD at the back that has listening exercises, you can also find these online.
The first textbook goes over both the hiragana and katakana alphabet. Each chapter has a list of new vocabulary words written in hiragana, katakana, and kanji. Before starting the chapter I would suggest trying to remember the vocabulary words, how to write them and how to say them. To do this make flashcards or practice writing the words over and over again until you memorize it.
Each chapter corresponds with the workbook which helps you practice with writing Japanese. The workbook reinforces what you learn throughout the chapter by letting you create new sentences, answering questions in Japanese, and using the CD for listening exercises.
To utilize this textbook to it's full potential I would suggest studying 1 chapter at a time, don't overload yourself with more or you won't be able to retain the information. To cut down on stress try to learn at least 1 chapter a week, by studying a page per day and filling out the corresponding workbook pages. Once you feel confident that you've learned the chapter move on.
Every so often try writing a paragraph or a page in Japanese incorporating everything you learned in the chapters that you've learned so far to see if you really absorbed the material.
The Genki textbook and workbook can be ordered on Amazon for about 70$ USD.
HelloTalk is an app that can downloaded on an Iphone or Android. It gives you a chance to test your Japanese on real Japanese natives. Actually talking to Japanese people is the best way to retain your knowledge and learn more about the Japanese language. The app allows you to display what languages your proficient in and pick what languages you would like to learn. You can pick what gender and what between what ages you would like your language partner to be.
After you're paired with your conversation partner you simply start talking about anything. Throughout your conversation your conversation partner can correct your grammar, and you can correct theirs. The app also allows you to translate what your partner has said into your native language. The app has a lot more awesome features, including recording yourself talk and sending it to your partner, and even saving grammar corrections that you've received and your translation history.
Other Internet Sources
There a ton more internet sources that are free and easy to use, but I find it easier to have a physical book and workbook that allows me to really test my knowledge and that I know is credited.
You can try also looking about he JLPT website that has sample quizzes and online tests, however, these sample tests never change so it's easy to learn the answers and not really learn anything. The JLPT is the official Japanese proficiency test, they also provide textbooks but it's made to prepare you for a test, so it's a little bit stressful.
Heres the link to the JLPT website http://www.jlpt.jp/e/
I hope you found this article helpful on my personal experiences on learning Japanese on a budget, I wish you luck on your Japanese journey!