1. Your OS has a video card driver?
2. Your OS has implemented a HAL? (Hardware Abstraction Layer?)
If #1 is not met, but have #2 in your OS you can start by supplying a VESA compliant driver. If both are not met, better start working on a driver interface and hardware abstractions. You can always just make VESA driver part of the kernel without a driver interface however I don't see the OS going far on that road.
You should switch to protected mode inorder to address more than 1MB limit [upto 4GB] and to see those fancy graphics and also as Ryu said,you should also implement a video driver[VESA,...].
Take a look at http://www.mega-tokyo.com/osfaq2/.