If you have your own authority, or operate in a situation that allows you to source your own loads, you can start by knocking on doors or making calls to the people you wish to provide service for. Personal relationships take time to build, but can go a long way to making your business profitable. Once you get yourself established with a few good customers, it will be easier to get more. The contacts you make within these companies may not be able to give you any work today, but down the road they may be working someplace else and will remember you and could give you the business then. There aren't any magic tricks to developing an overload of customers overnight. Many of the brokers out there worked for years in dispatch, or other areas in the freight business, and took their knowledge and contacts and established themselves as brokers. Some of them do a good job, some dont, just as not all carriers live up to their promises.
As far as online tools go for connecting directly with shippers, try
uShip - The Online Shipping Marketplace - Ship Freight, Furniture, Cars or Moves. It can be an interesting site, as you can see who else has bid on the loads you bid on, as well as their actual bids. It becomes pretty obvious in a hurry how many low life, rate cutting fools, are out here operating, and lets you know exactly who they are. Many carriers as well as load brokers use this sight to get loads from one off customers, and with any luck you can build more business with the customer from there.