Hi, welcome to the forums.
If you're wanting to personally develop an iPhone app, then you need to learn the objective c programming language. You'll also need to buy into the 99$ developer account that apple has to offer so that you can put your app on the store.
As to your question about whether or not to hire a developer team for your app, my answer is this: first at least try to learn how to develop your own app, without hiring anyone. If you simply can't get anywhere, then you can start thinking about hiring developers. But be warned, some of these teams (not all though), can be quite expensive.
Also, just because a professional team created an app for you, does not guarantee that the app will be a success. There are many, many apps that were created by a professional development team and completely flopped. On the other hand, there are also quite a few apps created by one person with basically no budget, and do decently (ever hear of iShoot?).
As for how much an app would cost, it depends on if you're going solo or not. If you hire a team, it could cost thousands of dollars. If do it alone, then you could do it with virtually no budget, aside from the 99$ to get the account. Another great option if you have some money to spend is to just hire a single artist to create some artwork for your app. It costs much less than hiring a whole team but the resulting app could be just as good. I've never done this but I bet there are people on this forum who have.
That's my two cents, best of luck
