You should 

1) git pull
2) Work on your project
3) git add --all .
4) git commit -m 'your commit message'
5) git push
6) Now you may have a conflict, so you will have to 
7) git pull
8) See 
https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/resolving-a-merge-conflict-using-the-command-line
for dealing with commit conflicts