Rest and Resupply

Rest and Resupply is an unmarked quest in Fallout: New Vegas.

Quick walkthrough
Do enough quests on behalf of the NCR to become Liked with this faction.

Then visit Colonel Hsu at Camp McCarran who tells you he is pleased with your progress. As a reward he will give you a key to the NCR Ranger safehouse, where you can rest and restock.

Detailed walkthrough
If you are planning on completing the game the NCR way than this can easily be achieved by completing main NCR quests such as For the Republic, Part 2 and You'll Know It When It Happens and doing many side quests that give you NCR fame such as Restoring Hope. You will gain large amounts of NCR fame raising your reputation with this faction until you reach "Liked".