I'm trying to push a file to a git repo of a friend but errors on public key.

```
git push origin testbranch
Permission denied (publickey).
fatal: Could not read from remote repository.

```

Where and how do we define public / private keys? 

`git remote -v` returns:

```
origin  git@github.com:Sesamzaad/NET.git (fetch)
origin  git@github.com:Sesamzaad/NET.git (push)

```

Any help is appreciated.