This is a great question and one that I get asked at least once a week from entrepreneurs who want to build an app. I have a few quick words of advice:
1. You don't need a buisness plan, but a well though pitch deck is key. Guy Kawasaki has a great pitch deck plan: http://guykawasaki.com/the-only-10-slides-you-need-in-your-pitch/
2. Whether you are an novice or an expert, put your idea to paper. This can be in the form of user stories, bullet point features, hand drawn mock ups, screen shots from other apps - basically something to communicate to a developer what your idea is all about.
3. You need to spend some money to raise money. It doesnt have to be $100,000, but if you are serious about your idea and working with an app firm, be prepared to spend at least $5K to hire a team to flush out your concept and work with a UI/UX designer so that you can then present your idea to an investor. It is well worth it. My company has done this successfully for many start ups who then go on to secure a seed round of funding.
4. Your idea is not great. Everyone has "great" ideas. Its the execution of the idea, how thought through your idea is, who you end up choosing to work with to bring your idea to fruition.
5. Build an MVP to prove your assumptions and that your "great" idea has market value and you can gain users or customers. Dont focus on being everywhere at launch, focus on 1 platform (iPhone) and build it native. Pivot when you need to, be ready to abandon everything if you can't prove it. Read the "Lean Startup" before you do anything else.