Uppercase or lowercase style which do you prefer as a programmer (comments, filename, username..)? [on hold]

Uppercase or lowercase style which do you prefer as a programmer
(comments, filename, username..)? [on hold]

I am a programmer from China. I would like to learn English, so I would
like to keep my work environment English as well. There have been many
problems; one such problem is writing comments in code. This always
confuses me.
Should the first letter be capitalized or not capitalized
// do something
or
// Do something
I read a lot of books and source code and found that they follow no strict
guidelines.
As another example, when creating a folder, I wonder if the end should be
written as "Project" or "project". I think writing it as "Project" may
look better, but it is faster and more convenient to write it as
"project."
I hope you can help me, or talk about your specific habits and give me
some ideas.