Hi
Well, Humberto gave a very good and detailed answer and I will be happy to add just a little bit more.
First of all - I must disclose that I work for a company that develop apps and other software for every one, from people like you that are just starting to big well established companies.
When someone comes to us with an idea, we usually sit with him and write down its specifications. We write in details what the app should do (and look like) from the users' side (with some mock screens) and from the server's side. Once we have that, we can estimate the scope of work and give a price offer.
This cost money, so be prepared for some initial investment from either your own pocket or from what is known as the 3F (Friends, Family & Fools...).
After you have the specifications you can start asking for price offers so you will know how much money you need to raise. A good way to get investors would be to develop a demo or proof of concept. Something that will look like an app but will not do much. Developing a demo is much cheaper and in most cases will help you better convey the idea to potential investors.
Good luck and give me a call if you have more questions or want more ideas.