Need help painting a desk top made out of MDF
MDF is essentially a sponge when it comes to painting it. So the first thing you need to do is prime it properly so that it doesn't absorb the paint. To prime it properly, use a non-water-based primer. These are known as solvent-based primers, or, more commonly called "oil-based" primer. 

If you use a water-based primer, you end up with the sponge problem...the MDF will absorb all the water and your surface will end up with a rough raised-grain texture that you can't easily sand out.

Once you get a oil based primer on the MDF, you can then paint it as you wish. 

As for resilience, note that MDF is soft. So no matter what you coat the surface with, you are going to be at the mercy of the substrate when it comes to heavy objects hitting it. 

Fo starters, use a good quality paint. If you are particularly concerned, consider one of the epoxy based paints used to paint over laminate countertops/kitchen cabinets.

You may ultimately want to consider just putting a laminate on top of the MDF or just replace the MDF part with a section of countertop. Remember that MDF is soft, so that might be the weak part of the entire equation