The main thing that you need to ask yourself, is "What can i offer that is equal or better than theirs."
This is a common misconception that many people overlook. You have to think about this because if you don't, then you can start up your business and then find out that someone is offering something better than you and that all your "Possible Clients" are going to them.
You need to think about what you can do to make your service, just that "Little bit" better than those who also offer the same product/service that you are offering/selling.
Once again, some people will think that they can just setup an online business and then people will just come rushing to them.
Running an effective and worthwhile online business takes time and a fair amount of effort.
"Do not give up your day job", because you may not profit much for a while until your company/store is well known and is trusted by many.
Once you have your idea, take a look at who offers the same or similar product/service.
Once you have done your research, see if you can make better changes or offer something cheaper but make sure that you can still cover your costs.
"Do not rush!"
If you start rushing ahead of things then the chances are that you may miss something vital. Even minor things can have a massive affect on how it works and if it succeeds or not.
Take things slow and think through them many times. "Review your thoughts"