Open a support ticket on the A2 Hosting Customer Portal at https://my.a2hosting.com. In the ticket, please provide your license key. We must have this information to complete the installation process on your server.
Do not try to install the New Relic agent yourself. The agent installation process requires root access, and must be done by A2 Hosting support.
After A2 Hosting support completes the installation process, you can use the New Relic Infrastructure dashboard to monitor your server.