Here I'll share a bit of my experience about how to learn \LaTeX
from zero, the best editor for \LaTeX
(IMO) as well as typical mistakes.
Table of Contents
"Who Is LaTeX" Stage
Typing
It is a requirement for any programming activity. Open any trainer for blind printing with ten fingers — and go ahead!
How To Learn

I recommend this free online introduction to
\LaTeX
in three parts, made by Overleaf. Instead of a dry book guide, they made it as a presentation. There are a lot of examples, with a focus only on the most important stuff. This introduction should take about 4hours for you to read.Also, Overleaf has great docs. Later, when you have some questions about
\LaTeX
, you can look it up in their docs. 
Videos are usually the easiest way to learn, and for a quick start in
\LaTeX
take a free course from the HSE (in Russian).The basics could be learned in 45 hours, and two days off are enough for the entire course. Video lessons are better than thick books in the beginning.
Where To TeX
There are two ways.
 Online compiler like overleaf.
 Very simple.
 Realtime online teamwork.
 Set up your environment.
 So much faster compiling!
 Takes time.
 Easier to integrate Git.
GUI / Text Editor for LaTeX
The best option, in my opinion, is LaTeX
in Vim
. There are gold articles in which Gilles Castel shows his Vim
+ LaTeX
setup and how it works.
Otherwise — you can try TexStudio
, but it looks like an ancient artifact.
Also, there is the texpad
app for macOS, but I had some issues with buggy behavior (in 2020).
PS: The Vim
+ LaTeX
setup can easily take 20+ hours to make it work if you are new.
Life Hacks

Use
\newcommand{command_name}{script_of_command}
for commands you use very often.Compare
f: \mathbb{R} \to \mathbb{R}
andf: \R\to\R
.
$f: \mathbb{R} \to \mathbb{R}$ Learn how to TeX in
Vim
=)
Typical Mistakes

Please, God, don't do 500+ lines main.tex...
 It will be an unmaintainable mess.
 You don’t have a universal preamble.
Instead  use
input{anotherFile.tex}
orinclude{anotherFile.tex}
to structure code. 
Not using or defining
LaTeX
environments.
For every theorem, proof, note, etc. you can set special environment (\begin{theorem}
,\begin{proof}
). Now you can change the style of your theorems by changing one line.

Not using
multiline
oralign
environments for long formulas.
$\begin{align*} a &= b + c + d  d \\ &= b + c \end{align*}$\begin{align*} a &= b + c + d  d \\ &= b + c \end{align*}

Not using
\cfrac
or\displaystyle
when necessary. Otherwise, the formula may get hard to read.
$\frac{2 + \frac{8}{4}}{6}$  without
\displaystyle
$\cfrac{2 + \frac{8}{4}}{6}$  with
\displaystyle

Not using
\text
inside formulas.
$\begin{align*} a &= a + b + c + d , when\; sun\; is\; red \\ a &= b + c, \text{ when sun is white} \end{align*}$\begin{align*} a &= a + b + c + d , when\; sun\; is\; red \\ a &= b + c, \text{ when sun is white} \end{align*}

All variables inside text should be math mode.
a or $a$

Avoid making new lines in text with
\\
. Using empty lines instead makes code much more readable.

Not using
\ldots
for ...... or $\ldots$

Ignoring warnings and errors.
 You must not ignore erros.
 Warnings help you deepen your knowledge and improve code.
Not using
Git
for big projects.
Best of luck...Using tutorials from ancient times.
You can meet outdated and deprecated practices.
Useful Links
 Get LaTeX code from formula screenshot  super ultrauseful
 Draw symbol and get latex command  ultrauseful
 Latex table constructor
 LaTeX Cheatsheet
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