Most of the resources listed here I have personal experience with and can more or less vouch for.
Background artwork by あるかとぴあ on Pixiv.
It is instrumental that learning the kana (i.e., hiragana & katakana) is the first thing you do after deciding to learn Japanese. These are two sets of 50-ish symbols each that represent the sounds Japanese syllables; as for why you need to know two symbols for each syllable: why do you need to know both 'A' and 'a'?
Put in an hour or more a day with any of the tools listed below and you will have them all memorized in a few days' time.
DJT Kana Itazuraneko Kana (Mirror via wimmer.cool) Pretty much the same as DJT Kana above, but uses a dark theme. Real Kana More or less the same as the two above, but arguably less straightforward.It is recommended that you pick one guide from each category and read it back-to-back without dwelling on the tougher concepts too much. After your first read-through, you can go back to revise (it will be easier to understand as you will have more familiarity with the language). If you find anything to be particularly complicated, reference a different guide.
Anki is software designed to help you memorize things quickly through the process of spaced repetition. This is by far the most efficient and effective way of learning Japanese vocabulary.
I personally recommend getting the Core 2k/6k deck and going through it at a rate of 20 new cards/day in the beginning. Once you reach somewhere around 2000 cards (or whenever you start feeling like it's going too slow), bump it up to 50 new cards/day (but bear in mind that this also means more revisions).