This is how I would do it (in fact, we are doing something similar for a client right now):
Step 1: Research
Learn about your market, other existing networks and how you can create a better service.
How are you going to make money out of your site? Give some thought to this. Are you going to sell it, sell ad space, charge for a premium membership or sell products/services to your members?
Step 3: Create the Website
It will take you a few months so get started as soon as possible.
Step 4: Start Preparing Your Marketing Campaign
Start building your lists, Facebook contacts, Twitter followers. Start participating in forums, blogs and discussion boards. Contact list owners and give them a reason to promote your new network. Don’t forget about your off-line marketing plan. Think about 1) Who is your target market? and 2) Where can you find them? (magazines, websites, radio stations, etc.)
Step 5: Launch Your Marketing Campaign
Launch all your campaigns at once. Your community has to grow extremely fast at the beginning. Encourage people to invite their friends to participate.
95 percent of the success depends on your ability to grow the network quickly at the beginning. If you get the snowball rolling down the hill, it will get bigger and bigger. The initial push is your main challenge now.
Let me know if you have more questions.