Clone your github repository and add commit push your changes

1) get your repository URL from your github account: e.g. url1

2) Open terminal and type
> git clone url1

this will create the local copy for already existing remote repo.

3) Start working and create some file in the cloned directory dir1 and add the file to local git
dir1> touch file1
dir1> git add file1

4) Commit the changes with some message
dir1> git commit -m "first commit"

5) Push the changes to remote server
dir1> git push
.

6) Check your github account for the changes, it must be reflected there.