Cannot push to GitHub - keeps saying need merge

I'm new to GitHub. Today I met some issue when I was trying to push my code to GitHub.

Pushing to [email protected]:519ebayproject/519ebayproject.git
To [email protected]:519ebayproject/519ebayproject.git
 ! [rejected]        master -> master (non-fast-forward)
error: failed to push some refs to '[email protected]:519ebayproject/519ebayproject.git'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Merge the remote changes (e.g. 'git pull')
hint: before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

I have not pushed anything in the repository yet, so why do I need to pull something?

This can cause the remote repository to lose commits; use it with care.

If you do not wish to merge the remote branch into your local branch (see differences with git diff), and want to do a force push, use the push command with -f

git push -f origin <branch>

where origin is the name of your remote repo.

Usually, the command refuses to update a remote ref that is not an ancestor of the local ref used to overwrite it. This flag disables the check. This can cause the remote repository to lose commits; use it with care.

As the message tells you,

Merge the remote changes (e.g. 'git pull')

Use git pull to pull the latest changes from the remote repository to your local repository. In this case, pulling changes will require a merge because you have made changes to your local repository.

I'll provide an example and a picture to explain. Let's assume your last pull from origin/branch was at Commit B. You have completed and committed some work (Commit C). At the same time, someone else has completed their work and pushed it to origin/branch (Commit D). There will need to be a merge between these two branches.

local branch:                         --- Commit C 
                                    /
                                   /
                                  /
origin/branch: Commit A ------ Commit B ---- Commit D

Because you are the one that wants to push, Git forces you to perform the merge. To do so, you must first pull the changes from origin/branch.

local branch:                         --- Commit C -- Commit E
                                    /               /           
                                   /               /             
                                  /               /               
origin/branch: Commit A ------ Commit B ---- Commit D 

After completing the merge, you will now be allowed to fast-forward origin/branch to Commit E by pushing your changes.

Git requires that you handle merges yourself, because a merge may lead to conflicts.

Have you updated your code before pushing?

Use git pull origin master before you push anything.

I assume that you are using origin as a name for your remote.

You need to pull before push, to make your local repository up-to-date before you push something (just in case someone else has already updated code on github.com). This helps in resolving conflicts locally.