Sometimes the service you are thinking of building might already be available - just hidden in fine print. Sometimes it's just a "promotion" that might end at any time, but even so, you might be able to use it to build something better. A good first step is to evaluate the market - to see what people are already spending or willing to spend, and if you can build something better - and what you can charge for it. You don't always need to charge less - sometimes people are willing to pay more, for something that is a lot better.

Types of service available - 

Where is the closest tower, or fiber connection point?
What is the cost?
What is the cost for the connecitivity you will need to serve 100 concurrent users?
What is the cost for the connecitivity you will need to serve 1000 concurrent users?

Things to learn about:
    Contention ratio means how many people share the connection. For example, ADSL contention is 1:25 which means 10Mbps is shared between 25 People. 

Approximately 30% of a network is online at any time - this varies, but a 1:3 contention ratio is fair. 
Anything worse will offer poor speed.
People need reliable networks - if they can depend on it they will, and they will use it before another. 
If they can't depend on it, then if they have problems they will just assume its you and find someone better. If you want to be successful, be the best - by getting the best to help you.