What programs do people use to make clothes for Poser figures?  I don't know where to get started.

I know that Aquila over on the Daz fourms is making a Space Marine outfit for The Freak using Anim8tor (Free).
Or you could use Wings3d (also free)
I personaly like Hexagon(bought from Daz), even with it's bugs and other problems.
Then There's Silo (bought from Nevercenter). No comment on this one never used it although the interface looks VERY userfriendly.
Blender (again free) has a very steep learning curve, I'm near the starting line and hope to move up to being able to move the camera soon, lol.
Then there are the heavy hitters, that heavy on the wallet, 3d Studio Max, Lightwave and Carrara. 

I'll assume you don't have much or any skill in modeling since from what i've learned, makeing basic clothing is infinatly easrier than modeling a person or animal. Although others might disagree. My first bit of advice, which I'm sure others will also give is basicly try out everything, most pay products have free trails. Though I'd say try Wings3d and Animator first. They may be free but that dosen't mean that they aren't powerful. They are both good for beginners.

 If you need tutorials on how to make clothes then check out the Hexagon Forum over at Poserpros. I believe that the tutorials will transfer to other apps easily because box-modeling is box-modeling.

I hope this long winded reply is helpful in some way.

Or you could check out PhilC's work. His clothing designer, helps design clothing for you, might help you get the basics.  http://www.philc.net/

Oh, I forgot about that one. Yeah you can't go wrong with PhilC's work.

*"you're worrying about step 792...we're on step 3 or 4"- Apollo 13...;)*granted, designing clothes is a rewarding pastime. Just be aware that it has a steep learning curve of it's own..;)

I have to stick with free things.  I can't afford PhilC's tools, although they look good.

I managed to make a simple pants obj, but at least it's a start.

Free, Wings3D, Anim8tor and Blender. Try them all and see which one works best for your way of thinking. I started with Wings3d because I tended to block model when I started and there is a great forum here for it. I have moved on to other apps but I still use wings for some things.

I always tell new users that modeling apps are very personal. Try all of them and see which works best for you and not what someone thinks is best for you because they like it. That is why there ar so many different modeling software out there.