One of my 'worries' is that I understand that the language is built up of 'four sets of characters, hiragana, katakana, kanji and romaji to remember' (I got this of the website
www.learn-japanese-adventure.com).
The two Kana scripts are based on the same consanant+vowel system but used for diferent purposes as mentioned above.Doesn't take long to get the hang of them. Especially hiragana as it's used for Japanese words, so used a lot. Don't stress about having to look stuff up - it will sink in as you read and learn more. I'm getting there with Hiragana, but still need to look up bits of Katakana now and then as it's not used as much in the stuff I'm learning.
Romaji or 'Roman- ji (ji = letters)' is just writing it out in the Latin alphabet. Useful as a tool, and whilst learning the kana. You can get tutorial books in romaji, but that would be like like learning Russian in the Latin alphabet - possible but misss the point and causes more trouble than its worth in the long run.
Kanji is, as mentioned above, a big task. Books like Genki are largely in hiragana (with katakana where appropriate). But start to introduce Kanji early on. However, if you start learning Kanji as an extra specific task along side your main textbook you get both used to them, and build up your vocabulary of them as you go, rather than having to learn them all later on.
The problem here is that whilst litle kids books and textbooks are mostly in the kana, everyday stuff like signs, newspapers etc. are mostly in Kanji with the grammatical extra bits in hiragana etc., so if you didn't study Kanji you would know from your textbook lessons that the word was a polite past tense verb, but not know what the actual verb was!
The good news though is that they reckon if you learn the first 1000 Kanji (about what Japnese kids get taught at school) you can probably understand 90% of what is written. Seems a lot, but Chinese needs a lot more to reach fluency. The better news is that they are not as bad as you may think at first glance. There is often a pictorial logic to parts of them which aid the process of remembering what they mean.
Besides, you need to learn a lot of words to become fluent, but just a few basic words can be a great help even to a beginner ("Can I buy please" and pointing has got many people through entire trips.). Kanji is the same. Learn some key ones and your options expand dramatically.