Extremely hard. Especially if you want to speak it fluently, and be able to read it, too.
Just for starters, Japanese has three different alphabets, and at least five different ways to just refer to yourself as "I". You might be able to grasp the basics, but holy shit, being both fluent, and literate in it is extremely difficult. It's right up there with Arabic as being one of the hardest languages to learn if your a native English speaker.
lolwhat
I just wanna highlight this:
It's basically an alien language if all you know is English.
EVERYTHING is basically an alien language if you only know one, because you don't yet know how languages work, so the intricacies of your own look like normal order of business and you end up writing creepypasta like the above. Is there a difference between "goodbye, hope to see you soon" and "fuck off forever and die in a ditch"? This is politeness, too.
edited to add:
"aww, it uses freaky characters so I can't know how to spell a heard word or to read a written word".
I hear this about
English a lot, from people who only speak Russian. THIS IS NOT A PROBLEM THAT EXISTS FOR
NON-NATIVE SPEAKERS, you realize it as soon as you make the bare minimum effort to start learning, it's a bullshit weaselly excuse to never even start.
If you don't know the word, you don't know the word; look that bitch up. If it's written, look it up in a dictionary, if it's spoken and your gaijin ear somehow correctly identified the sounds (fat chance), look up its hiragana spelling in a dictionary.
Bummer, it sounds like it's probably beyond my ability to learn, but never say never I guess.
Go to a language learning site for a language you know and look at the questions people ask. "But why is it the wrong word here, it's in the dictionary; but why is it rude", why, why, why, it never ends. Then the native speakers twist their brains
inventing an explanation. It's a bit like explaining to that one reddit guy why he shouldn't have tried establishing masturbation rules with a roommate on a work trip. Rules are for lolcows.
Japanese is easy, because, despite looking wacky, it's suitable to the kind of media and learning infrastructure an amateur learner has access to.
First up, a warning: do not fall for anything "kanji-free, easy for beginners, tee hee". You're learning a new language wholesale, not a writing system for a spoken language you already know.
Based grammar guide. The guy is writing a more directed quickstart guide, but it's still in progress. Also exists as a crappy (doesn't preserve position on page) free app. Anyway, it tells you to learn the alphabets first, so:
Free alphabet app: Hiragana Cards. Actually supports both alphabets. This one makes you choose the correct character out of a set of 4, so use elimination to your advantage, brute-force it and git gud, then proceed to do harder quizzes in Kanji Study (below). Also download and fill in some trace sheets (I attached the ones I use). Mind the stroke order.
Free kanji app: Kanji Tree. Try learning to recognize the first 3 sets (440 characters), it should take you about a week to remember them all and build up a healthy shield against doomsayers. Inb4 "what about writing" - don't worry about writing kanji yet if your goal is to
understand written Japanese and maybe ask a seller a question. Writing is good practice to solidify the characters in memory and get a better idea of how they're constructed, but the stroke order is a lot of overhead for nothing until you know the words to use them in.
Good kanji / dictionary / alphabet app: Kanji Study. No ads, free beginner demo version, no time limit, one-time payment. The dev is good, plz no pirate. This one makes you choose the correct character out of a set of 8 and is less suitable for brute-force learning than Hiragana Cards.
Unsupported casual language learning app: Memrise. Feeds you words and phrases to get a feel for the language.
Do not pay for it, it's unsupported. Get a patched apk. Install a userscript for the web version or download an older modded apk to force typing tests only.
Shitty word app: Language Drops. Feeds you words 11 minutes per day for free and demands a horseshit payment afterward.
Do not pay. AFAIK it needs root access to patch it, so if you don't have root (I don't, I have a rare phone with an expensive screen that I'm afraid to brick), 11 minutes is enough. Set it to use Kanji in the options. Screenshot your daily words (list of all seen words is paywalled).
Meh grammar app: Bunpo. Feeds you words and grammar, no ads, only the test schedule is paywalled.

The community: lots of weebs, lots of weeb media. Secondhand manga is extremely cheap on yahoo auctions ($1 per volume) and has an insanely high story/effort ratio. Start with something you haven't read. It only takes the equivalent of one 300-page novel to go from decrypting hieroglyphics to occasionally looking up unfamiliar words.
Use a mail forwarder to deal with xenophobic japs and consolidate packages. The latter is useful if your country has a high non-taxable threshold, or a low threshold and a minimum fee (with a medium threshold, ensure that the service
allows makes you fill in the customs form on your own, hur hur hur).