Start with the standard stuff: Objective-C and UIKit. You don't need any frameworks beyond this unless you are building a game and there will be plenty of time to fight with other poeples code when you are adding ad networks, analytics, in app purchases and so on without adding frameworks into the mix.
Objective-C and UIKit are quite large (Just Google NSString and look at all the methods on this basic type) and you need to learn as much of the standard stuff as you can. It's going to take some time and a lot of effort to be able to crank out polished apps in weeks instead of months.
If you are planing on building games then I would still recommend starting with the basics. Make a UIKit based app first. Then try your hand at a simple 2d game with box2d and cocos2d. If you are going to do 3d don't go with Unity right out of the gate. Go make some "simple" stuff using OpenGL ES direcectly. Good luck!
It completely depends on what you're trying to do with the app. A game with 3d graphics (unity)? 2d graphics (box2d, cocos2d)? Multiplatfrom in javascript(phonegap)?
This isn't very much information to make a decent suggestion, but I will make some assumptions.Assuming that data entered will be stored in the cloud, I suggest making a quick prototype with html/javascript and jqtouch (important for the native look and feel), and you can bundle it up with phonegap (or Titanium can do this maybe, I haven't looked into that).
This is even better if you're still in the testing or market research phase. This way, you don't even have to make it into an app, just host the html and js somewhere and run it in the browser while you show it to people. This allows for rapid changes and iteration, with the zero barrier to entry (aside from a little html and javascript knowledge). If it looks like you want to go forward, it's that much easier to make a native app because you already have the look and feel figured out. This also makes your wishes very clear in the event that you shop the development out to someone else.
Objective-C and UIKit are quite large (Just Google NSString and look at all the methods on this basic type) and you need to learn as much of the standard stuff as you can. It's going to take some time and a lot of effort to be able to crank out polished apps in weeks instead of months.
If you are planing on building games then I would still recommend starting with the basics. Make a UIKit based app first. Then try your hand at a simple 2d game with box2d and cocos2d. If you are going to do 3d don't go with Unity right out of the gate. Go make some "simple" stuff using OpenGL ES direcectly. Good luck!
PS. If you are ever in SF stop by our weekly iOS meetup: http://www.meetup.com/iphonedevsf/