To very easily create the much needed .gitignore file, simply execute this command in the terminal when inside the project folder (root of project):
echo -e "build/\n.DS_Store" > .gitignore
Do a cat .gitignore to check that the file indeed has build/ and .DS_Store on two separate lines, then you are done. I suggest you do this before your initial commit, as these files are likely in your project folder already.
Posted by Johan