ALWAYS PULL BEFORE STARTING WORK ON A REPO. If you aren't up to date with the repo and make changes and then try to push them up to GitHub you're going to have a bad time. AKA merge conflicts
Here's how you pull down everything from the repo
git pull origin master
After completing what you're working on, run these 3 commands to push your work up to the repo. (You must be in the root folder of the project you're working on in order to run these commands.)
git add .
This will add all the files you worked on to then run the next commandgit commit -m 'YOUR COMMIT MESSAGE'
Write whatever you want the commit message to be inside the quotesgit push origin master
This is the command that will push everything you've worked on up to GitHub so that others can then pull down what you've done to their local environment.