Mobile or Desktop App? How should I start?

I would think that they market the you are attempting to target would dictate what the best platform to start would be. Without knowing your market or idea, this is impossible to say.

Both mobile and desktop apps have their benefits and traffic/search opportunities, but only if targeted correctly.

I am not sure if this helps, but without more info this is the best I can do. I am willing to chat more, if you would like.

Take care.

Mobile apps are the trend, people will need accessibility, if you are able to have one done, you can also have the other done simultaneously. It should neither cost more or require advance code/architecture changes.

I recently came across an excellent post from someone struggling with that same question. I hope it helps you: http://blog.cloudfour.com/responsive-design-for-apps-part-1/

Although I am currently writing a book later out this year, my "day job" is running a creative agency where we've created several responsive websites, including http://newportdunes.com and http://searchoptimizers.com I find that sketching it out first allows for the constraints of the design to focus on what is most important. My favorite tool for this is UI Stencils responsive sketch pad, found here: http://www.uistencils.com/products/responsive-sketch-pad

I know you are specifically asking about an app and not a website, but I think it can still be a useful tool in the "fingers first" thought process and decision process. In Jason's post, he recommends Appcooker.com as a ipad based tool.

I think you'll find that Mobile or fingers first is a fantastic mindset with little downside as it forces you to get clear on the context of your content, size targets appropriately, and then the desktop design does not suffer for it. We sometimes go the other way, too, but we understand what "fluff" we give up when moving to the mobile side. Google Luke Wroblewski for more excellent info on his Mobile First way of thinking.

Good luck and let us know when your prototype is up!

There is a huge difference between the two including capability/functionality, target users, use cases. If you do not know from which one you should start, you probably need to do more homework before getting into development. Let me know if you need some points to assess in order to make the decision.

Some of the other answers here missed your true question of _what_ should be evaluated. I guide my clients around this exact problem. Even though the focus of my business is native apps, I have no problem saying "you don't need an app for that". With more details I could help you further, but in general you're asking:
Who (is using the app) - millennials and retirees use devices differently
Where (is it being used) - is it mostly going to be used on mobile? is it an extension of something else
What (kind of experience) - Web/Desktop/Mobile all have different UI conventions and limitations

That's really just the beginning! As you understand the idea better and the targets, it should become more obvious which is the better choice. If you want to call me, I can provide more detailed suggestions about an appropriate route to take with more information.

Identifying your requirements is one of the most essential best practices in app development, mobile or otherwise. Carefully research the targeted capabilities to determine if they are achievable in your mobile web app. Another common gotcha for mobile web app developer newbies is to assume that web-based code for a desktop browser will work “as is” in a mobile browser. The HTML5 tag’s auto play functionality, for example, does not work on mobile browsers. Similarly, the CSS transition and opacity properties are not supported in most mobile browsers nowadays. You will also have problems with some web API methods on a mobile platform, such as the SoundCloud music streaming API that requires Adobe Flash which is not supported on most mobile devices. A common gotcha for mobile web app developer newbies is to ass-u-me that web-based code for a desktop browser will work “as is” in a mobile browser. A particularly complicating factor in mobile web application development is that the lifespan of mobile devices tends to be much shorter than that of desktop displays. These shorter device life spans, accompanied by constant releases of new mobile devices and technologies, yield an ever-changing landscape of to-be-targeted devices. For a mobile web app, it is important to be aware that a Retina display makes low resolution images look fuzzy and pixelation can occur. The best app development solution in these cases is to have the server recognize that the request is coming from a Retina device and to then provide an alternate higher resolution image to the client. For example, in iOS 6 and above, there is no support for the navigator getUserMedia functionality since the camera is only accessible through native apps. Two great resources for checking what is supported on specific devices and browsers are caniuse.com and html5test.com. It’s unrealistic to expect that transfers will always be done over a WiFi network, and you should know that 60% of mobile web users say they expect a site to load on their mobile phone in 3 seconds or less . 60% of mobile web users say they expect a site to load on their mobile phone in 3 seconds or less.
Unless you have a highly peculiar set of requirements, it is likely that use of one of these frameworks will reduce the level-of-effort to design and implement your mobile web application. Just enter the URL for your application, select the browser, version, and operating system, and you will get the emulated view of your site in that environment. Native apps are written in platform specific languages using platform APIs, like Java or Kotlin for Android, and Objective C or Swift for iOS. A mobile web application is a website optimized to be used on a mobile.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath