You can use either the stock tools or an app. If you use stock tools then what you want is inside the Sharing Pane in System Preferences. Turn on screen sharing.
The problem with the stock tool is that it only works if you're logged in. So, you'd need to make single user login mode with automatic login active in case you ever reboot.
I use OS X VNC, but it's a bit hard to find now. Unfortunately, at the moment I do not have access to it so can't post it up. Others may have a link or a different suggestion. OS X VNC is now called Vine Server though so maybe you can track it down that way.
The difference is that the app allows you to install a kext that automatically starts the VNC server as the Mac boots. It means you have access to the end of the boot sequence and the login screen.
EDIT: There you go,
@Amethyst1 has the link.