Addendum: After finally delving into a proper grammar guide, I can confidently say it’s where you should start your adventure with Japanese. At the very least, reading a bit about what kanji are, how they are structured, where did they come from and how did they come to be like that will help you a lot in learning them in the future. https://www.imabi.net/
You know, stuff like “all verbs end with u”, “the kana in the verb modify the meaning” and “I-adjectives’ with さ in place of い are nouns”
“all verbs end with u” - 言う, 引く, 足す, 立つ, 休む, 来る (but not all words ending with u are verbs 全く)
“the kana in the verb modify the meaning”
見る - to see
見える - to be visible
見せる - to show
分かる - to understand
分ける - to separate
(often changing the object of the action)
上がる - to rise (yourself)
上げる - to raise (something else)
代わる - to replace (yourself, substitute for someone, etc.)
代える - to replace something (like a broken vase)
“i-adjectives’ with さ in place of い are nouns”
There are two types of words that can be uses as adjectives in Japanese, often referred to as i-adjectives and na-adjectives, or adjectives and adjectival nouns respectively. The former are regular adjectives, and in their basic form they always end with い, hence some grammar books call them i-adjectives. The other group are nouns, some of which can be used as an adjective, with a -な attached at the end, hence na-adjectives. (Like the word 馬鹿(ばか - baka), which can be used both as a noun meaning idiot, and as an adjective meaning stupid) (In their basic, noun form they can also end with い, as do other types of words, so you shouldn’t assume that all words ending with い are i-adjectives.)
長い - long
長さ - length
高い - tall
高さ - height
大きい - big
大きさ - size
(but not all i-adjectives’ noun form has さ at the end)
白い - white (adjective)
白 - white (noun)
赤い - red (adjective)
赤 - red (noun)
Edit: Even more basic stuff, but I though it’s a good place to mention it.
Most kanji have several spellings, separated into on’yomi (borrowed from the Chinese language, which, despite only having a single spelling for each kanji, still evolved throughout the ages, and the Japanese language, which kept borrowing new words from Chinese over the centuries, ended up using several versions of it for different words), and kun’yomi (from old Japanese words, that had a similar meaning). There’s also nanori used for old Japanese names, but I’ll skip that one. Most of the time, on’yomi is used when you combine several kanji together, like when you want to say “sea fish” 海魚(かいぎょ), while kun’yomi is used for separate use, like when you want to say just “fish” 魚(さかな), though not always. Apart from nouns, both versions can also appear as the core (the kanji part) of adjectives, “black” 黒い(くろい), adverbs, “first of all” 先ず(まず), verbs, “to see” 見る(みる), etc. With WK you mostly learn on’yomi as Kanji items and kun’yomi as Vocab items (though not always), so don’t be surprised when you run into situations when you learn the spelling for a kanji as a Kanji item and then see it with a completely different spelling as part of a verb later on.
Not all expressions with multiple kanji use the on’yomi, “this month” 今月 (こんげつ), “every month” 毎月(まいつき). Similarly, there are single-kanji words that use the on’yomi reading, like “king” 王(おう), “rain” 雨(あめ), or the numerals. There are also some Kanji that originated in Japan, and might not have an on’yomi reading, as there are old Japanese words, that use multiple kanji, since they share their meaning, but not their spelling, resulting in irregularly spelled words like “adult” 大人(おとな), or “today” 今日(きょう), that use neither of the regular spellings, or the other way around, using the kanji for their sound, and not the meaning, both for Japanese and loaned words, “haphazard” 出鱈目(でたらめ), “America” 亜米利加(アメリカ).
There’s also a thing called rendaku, when combining kanji, where the spelling of one of them gets slightly altered, to make it distinguishable from the sound of the separately used kanji, and there’s no rule to it.
兄 + 弟 = 兄弟 (きょう + だい = きょうだい)
王 + 子 = 王子 (おう + し = おうじ)
人 + 工 = 人工 (にん + こう + じんこう)
To sum up, you basically have to learn the spelling separately for each word. Have fun 
Edit2 - Counters
Nouns don’t have plural form in Japanese, so you’ll often see numbers being used to express the count of things you’re talking about. However, in order to accentuate what are you counting, you need to add an appropriate counter at the end (and there are a lot of them). For example, the counter for people is ~人(にん), so three students would be 三人学生(さんにんがくせい), while three computers would be 三台コンピュータ（さんだいコンピュータ), ~台(だい) being the counter for large machines.
On that topic, a lot of words borrowed from English language, function in Japanese as proper Japanese words written in katakana, like コンピュータ(computer), レストラン(restaurant), or タクシー(taxi). Though they sound alike their English parents, they are proper Japanese words, and you should learn them separately, as in, a Japanese person who doesn’t speak English will not understand you if you say “computer” instead of コンピュータ(konpyūta) even if they sound very similar. (ā, ē, ī, ō, ū are elongated vowels in transcription. They generally mean double vowels, though it doesn’t always mean that it’s the same vowel used twice. For example the kanji “ten” 十(とお - too) and “correct” 当(とう - tou) will both be written as “tō”)
Edit 3 - Homonyms and Accent
This one’s just the best. Not only do elongated sounds change the meaning of a word (ie - house, iie - no), but the sound you put the accent on in a word also does (ima - now, ima - living room)