Let me start with this…
For school and grades, there are many more than just "12" English grammar rules and tips that you "absolutely need to know".
On the contrary, when you're writing something online for the communication purpose (say, blogging), there's literally NO "rules and tips".
As long as you’re communicating in a clear manner and that the target audience understands you, there's no grammar rule and tips.
That said, here are 12 basic (and traditional) English grammar rules and tips:
1. When you use words like "however" "therefore" and "otherwise" to join independent clauses, you need to add additional punctuation after them.
2. There's a BIG difference between "They're", "There" and "Their". 'They're' is THEY ARE. 'There' is used when referring to location. 'Their' is someone's.
3. "It's" and "Its" are very different. 'It's' means IT IS. 'Its' is possessive.
4. "Whose" and "who's" are very different. 'Whose' is possessive. 'Who's' is short for 'who is' or 'who has'.
5. Do not start your sentence with 'And' and 'But'.
6. When you’re talking about an unfinished past, use present perfect (and NOT past perfect). "I have already worked for 2 hours today."
7. Semi-colons are used to join two ideas when conjunction or a full-stop makes those two ideas either too complex or incomplete to understand.
8. As much as possible, avoid ending a sentence with a preposition. Although acceptable now, the conventional rules are against it.
9. The number of the verb in a sentence is determined by the subject of that sentence.
10. Passive voice isn't something that's advised by the Grammar traditionalists. So, wherever possible, opt for active voice.
11. When using a verb phrase, the adverb usually goes after the first auxiliary verb.
12. There are certain words that are always capitalized. The first letter of the first word in a sentence. Proper nouns. Titles and abbreviations.
These are 12 very common (and traditional) English grammar rules and tips.
Of course, there's plenty more.
As mentioned already, unless you're writing for grades, stop obsessing over the grammar as long as you’re clearly communicating and that people on the other side understand you.