First of all, the first question, the specific steps are also very simple, roughly the following steps:

• Initialize warehouse

git init

• Add gitignore file

• Check the status of the files in the warehouse. Here you can see whether the files that should be ignored are ignored and whether the files that should be added can be seen

git status -s

• Add to staging area

git add .

• Submit

git commit -m "commit message"

• Add remote warehouse

git remote add origin <url of remote repository>

• Push to remote warehouse

git push -u origin master

The origin in the above command can be specified at will, it is just customary to write it like this.
Then there is the second problem. This is because those files have been tracked before, and modifying .gitignore again will not take effect (this seems to be a known bug of git). The best way to solve this problem is: if you have just made some modifications, submit these modifications first, and then run the following command:

git rm -r --cached .
git add .
git commit -m "gitignore已经生效"

So I modified .gitignore to exclude unnecessary directories and files again

You need to delete it first and then commit it. Next time you add a file, ignore will take effect