Talking to Yes Man will initiate this quest. He will explain Benny's plan to take over New Vegas, starting with killing Mr. House. You will also get the Wild Card: You and What Army? quest.
Killing Mr. House
On the wall near Mr. House, there is a terminal which allows you to unlock a hidden door to the antechamber. To unlock the terminal, you need 75 Science skill, the Platinum Chip (gained by either killing Benny or finishing Render Unto Caesar, or a Lucky 38 VIP Pass (found in the H&H Tools Factory or in Chief Hanlon's Desk at Camp Golf).
Once you unlock the antechamber, an alarm goes off and all the Securitrons become hostile, so be prepared for this. Go through the door, deal with the Securitrons and activate the terminal here to open the control room. Here, you will find Mr. House, and another terminal. Activating this one will open Mr. House's sterilisation chamber. You can talk to him, and choose to kill him or leave him as he is (he will die anyway because of exposure to contaminents).
Installing Yes Man
Return to Yes Man at Benny's suite in the Tops Casino. He'll tell you to meet him at the Lucky 38. You'll find him in the Penthouse, where you originally spoke to Mr. House. Speaking to him triggers an installation into the mainframe of the house. He will tell you to go to the basement, and once there, you will be treated to a demonstration of the Securitron upgrades.