One of the first things you need to do if you want to learn Japanese is to learn all the hiragana. Most textbooks include a quick intro chapter that basically says “here are the hiragana, memorize them before going on,” but for most people, that’s not going to be any fun or effective at all.
There are books (some of which are cute and fun enough to make the learning entertaining) devoted entirely to hiragana, usually with each one getting its own chapter, and with each one being accompanied by a funny picture to help serve as a mnemonic while you remember it. This way, when you are reading and you see one and think “oh, I know I know that one…” the mnemonic picture will pop into your head and you’ll remember it quickly. Over time, your brain stops relying on the mnemonics and starts instantly remembering each character instead.
Once you know all the hiragana (which can be done in less than a week with casual/lazy studying), you should then learn the katakana. Again, get a little book dedicated entirely to katakana and take a few days to learn them, using the funny mnemonic pictures to help you remember the ones that you struggle with.
Once you’re at this point, you can begin actually studying the language with a textbook.
Of course, you may want to do these at the same time, depending on your attention span. The point is, however, do not neglect learning the hiragana and katakana or your progress will be limited from the beginning.
When choosing a textbook, choose one that does not rely on romaji. It’s ok if it has both Japanese writing and romaji together (although you may wish to cross out the romaji to prevent yourself from cheating), but romaji-only textbooks are useles. Even if you know that “watashi no namae wa Yamamoto desu” means “my name is Yamamoto,” you will never see it written like that in real life, and you won’t know what to think when you see the same thing actually written in Japanese: わたしのなまえはやまもとです。
As far as learning kanji goes, it’s a little more involved than learning hiragana and katakana, so see the article about kanji for more info on that subject.