First I would search the net to see if there is anything like your idea out there. It's ok if there is, you just need to figure out how to make it better. People alway say to get the customer first and then build the site. By doing this you will know if you have a market for your idea before putting money into it. Make sure your website offers value to the user. Do you have a domain name in mind. Buy the name and get a company to host the site. If you know how to build a website then build it. If not then you can go to elance or odesk and list that you want a website built with detalis for free and people will bid on your job. Once you decide on a website builder then you are off and running. At some point you may want to consider setting up a corp or LLC. I'm sure other people will chime in but here is a list for now.
1) Search the internet for your idea
2) Get customers
3) get a domain and hosting company
4) get your site built