I want the Ranger Armor, but Hsu needs to give me the key first. So how do I get enough NCR reputation? What missions do I do?

Pretty much any mission for the NCR. There's also one for the Crimson Caravan that nets some NCR rep.

Check for missions at McCarran, Camp Golf, Camp Forlorn Hope, Bitter Springs, Camp Searchlight, Mojave Outpost.

You could also stealth kill a Ranger.

This. Visit just about any NCR stronghold and talk to the NPCs there, and they'll give you quests. Complete them favorably for the NCR and your reputation with them will go up.

Other locations not mentioned above include Helios One and the NCR Sharecropper Farm. You can also pick up NCR rep-boosting quests in Primm, Sloan and Boulder City.

I Thanks for the info, appreciate it.