Turn off the macbook main display while working with monitor
Using an external keyboard and mouse/trackpad
You can use Closed Clamshell Mode. This is a feature built in to Macbooks that allows you to use them like desktop computers, with the lid closed and using a monitor as your only screen.
You'll need:

An AC power adapter (like this one)
An external keyboard and mouse or trackpad, either USB or wireless (like this keyboard and this trackpad)
An external display or projector (like this one)
(Optional) A dock for your MBP (like this one)

To put your MBP into Closed Clamshell Mode:

Connect your keyboard and mouse/trackpad to the computer
Plug the computer into power
Connect it via HDMI or Thunderbolt to the external monitor
Close the lid
OR place a weak magnet next to your trackpad (the sensor for closed lid detection is there &amp; magnetic) for better cooling (I use an old iPad case)

Once in Closed Clamshell Mode, the Macbook's screen will turn off and it will still be outputting picture to the monitor. Read more on Apple's support page.

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Using the built-in keyboard and trackpad
You can dim your Macbook's screen all the way down (just press brightness down repeatedly) and then connect the Macbook to your monitor with HDMI. Your screen will be off but the keyboard and trackpad built into the Macbook will still be on and available. Optionally, you can download the free app Amphetamine to keep your Mac from sleeping.