To upload files to your server you need an FTP client installed on your computer. You can download FileZilla for free at http://filezilla-project.org/download.php. If you use Windows on your computer then download the Windows version (even if your VPS is Linux based).

For most Linux Distributions (e.g. CentOS, Debian, FreeBSD)

1. Open Filezilla
2. Click the "Site Manager" button and set up a new site
3. Set the "Host" to the main IP address of your server
3. Set the port to 22.
4. In the "Protocol" dropdown box, choose "SFTP - SSH File Transfer Protocol"
5. In the "Login Type" choose "Normal"
6. User the following credentials - 

User: root
Password: your server's root password

This will allow you to securely transfer files to your VPS.

IMPORTANT: NEVER USE PLAIN FTP WITH YOUR ROOT CREDENTIALS. ALWAYS USE SFTP INSTEAD.

Ubuntu

If you have an Ubuntu server, you will need to install OpenSSH Server. Do this with the following SSH command: sudo apt-get install openssh-server

For help, please see https://help.ubuntu.com/community/SSH/OpenSSH/Configuring 

Credentials to use are your ip address as the hostname with your user account and the corresponding password

cPanel Accounts

1. Open Filezilla
2. Click the "Site Manager" button and set up a new site
3. Set the "Host" to the main IP address of your server
3. Set the port to 22.
4. In the "Protocol" dropdown box, choose "SFTP - SSH File Transfer Protocol"
5. In the "Login Type" choose "Normal"
6. User the following credentials - 

User: your cPanel username
Password: your cPanel password

You can also create custom cPanel accounts for your websites.

For custom created FTP accounts use the following credentials - 

Port: 21
Protocol: FTP
User: user@domainname.com
Password: user password