how to start building and designing a web application

i have a little knowledge with designing web application and i'm confused where i should start ! just to explain where i'm lost in. i'm planning to build a website that have tow type of users they can register ,let's name them A and B . A can register and upload his location using GPS and B can use GPS to search for the closest A to them , also B can post photos , comments etc ..

i just give a general idea about the website and i want to make it using ASP.NET and C# . also the database should be in MySql .and i want to make a mobile app version in the future for the same DB. but now i'm confused about should i build everything by myself ? like designing the DB using Mysql SW and design the web interfaces using Visual Studio for ASP.NET and C# . then connect them togather and host my domain in arvixe.com and upload everything there ??

or i can desgin my DB and interfaces in my arvixe.com account ? excuse my english but i want a general guidelines what should i do ? what the easiest and efficient way to build this kind of web application?