You need:
-a good domain name
-a good brand
-a target of people
-a good SEO work (part of online marketing)
-a good brand management
-a good design
-some good advertising campaign
-a lot of dedication..
-a lot of patience..
you can do all this by yourself, it's not easy but it's not impossible either.. this list is just the base of a solid "pyramid" you can build.. read about every aspect first..