All I can say is its hard, very very hard, you need to know your coding and I assumed if you did you wouldn't be on here asking that, so its probably not the best idea for you but I'll help you out anyway.
First of all you want to decide which platform your going to run it on, whether its going to be OS (Mac) iOS (iPhone, iPod, iPad etc.) Android (Samsung/HTC devices) Windows, Xbox Or PS3. Now I take it you don't know much about this but to start off, get a mac, at the moment you can only develop apps and stuff for iOS & OS on a mac. You are going to have to download the SDK (Software Development Kit) for the different platforms your going to run the app on, you can usually just search the platform and then SDK after it on google to get what you want. (e.g Android SDK) Now I'm going to answer assuming your developing for iOS or OS, so what you're going to want to do is download Xcode from
Apple. Go here: https://developer.apple.com/xcode/
You are going to need an Apple ID (Its free) and then register as a developer. (Also free). Then you can sign in and download Xcode, its a big download due to all of the files and things to help you with developing, its 1.5GB to download so your going to have to have time on your hands.
Xcode is very confusing, if I go into it here, ill probably be here all day, I'm not joking! Google Xcode For Newbies, that should help you, or go the easy route and hire someone to do it for you, but as everything else in this world, it costs, varies who you go to but it costs A LOT.
Once you've done developing your app, you need to test it by clicking run at the top left of Xcode, this step is important so you can check for bugs (Errors) and fix them. Think of every way a user would use your app, do a lot of testing, then when you feel your confident about it, you can upload it to the Apple SDK community for others to check for bugs. You need to do this before you can upload to the app store. It costs though, $99. This is very reasonable because they need to make money out of this as well, and if your app is successful, you will easily earn that back in no time, don't worry though, they don't charge you apart from that. From there you can upload your app to the app store. (It wont go up straight away because Apple have to make sure its legit.)
You can tell how confusing it is just without me telling you how to do it.
Here is some guides that I found useful:
http://www.makeuseof.com/tag/develop-simple-iphone-app-submit-itunes/
http://www.creativebloq.com/app-design/how-build-app-tutorials-12121473
http://tinyurl.com/cja26cq
Hope I helped, I know its a lot of text, but you are going to have to get used to that as developing an app is basically just text.
Thats got to have been the LONGEST answer I've ever done,
That's a wrap!