1) Mock-up sketch of each screen on paper + specification sheets.
if floor price is equal or greater than database value for user ID session, then display new floor price.
And convert mock-up to digital schematic + spec sheets.
2) Code a prototype in HTML only (no visual design or logic) to ensure each page flows as intended based on the mock-up.
3) Code HTML prototype with logic and database (no visual design) to ensure each page works as intended based on the prototype.
4) Add in multiple user functionality and basic design to create a minimum viable product (MVP) that early users can use.
5) Add in payment functionality to create a minimum billable product (MBP) that early customers can pay.
6) Add in full custom graphic design from a graphic designer with mobile responsive design.
7) Release version 1 to market.
8) Package web app in a mobile app wrapper for Android.
9) Add more features based on market feedback and demands.
10) Consider building a mobile app for Android based on downloads and for other platforms based on device use analytics.
Each waypoint described makes it the build a lower risk and lower commitment as you get a result/outcome at each stage that can be given to someone at the next phase or use the same supplier — it also makes it far easier for suppliers to quote.
You would be looking at someone writing it in a backend web language like PHP, Ruby, Java, .Net, Python etc with a database like SQL, MySQL, MongoDB, SQLite etc.
This would change depending on the requirements in step 1 above and depending on how big the site gets it may need a rebuild later on.
In parallel with the above, it would also be worth doing the following to build demand and validate the idea:
1) Write the sales letter for the website's front page and the rest of the wording for all the pages.
2) Collect email addresses, phone numbers etc of early users so you have a hot market to launch to when step 4 above is ready.
3) Create opt-in lists for warm users based on keywords.
4) Create opt-in lists for cold users based on keywords so you can push towards to your hot list above.