We have tracked some files but then we need to ignore them. Here the steps to untrack them all:

1. Commit all changes

Run commit command to commit all changes including .gitignore file

git commit -m "update .gitignore"

Note: -m is paramater to add message to our commit

2. Clear all tracking

Run the following command to clear the cache. The command will only clear the tracking, not the files themself.

git rm -r --cached .

3. Re-track all files

Run the following command to stage all files and implements .gitignore file.

git add .

4. Commit all staged files

Commit all staged files by running the following command:

git commit -m ".gitignore fix"

Now, our repository is clean.

Source links:
Untrack files already added to git repository based on .gitignore