You need to clarify what you hope to achieve. You are free to start a competing crowdfunding site like any of the number that are currently out there. That, however, is radically different from what the JOBS ACT provided regarding investment crowdfunding. If you are thinking of that form of crowdfunding site, don't bother. It will be illegal until the SEC implements the rules.
This answer is for informational purposes only and is not legal advice regarding your question and does not establish an attorney-client relationship.
If you want the next kickstarter, go for it -- you won't make any money but hey, why not. If you're looking for an investment crowdsourcing though, see the answer from the other attorney here.
Also keep in mind that 99.99999999% of the time, if you've got a business idea for something on the internet, and you're asking for your legal advice on Avvo, it's going to fail. If you're serious, hire an attorney, get a real feasibility study and business model, talk to investors, and do it the right way. There can only be one Mark Zuckerberg.
The author is a Maryland attorney; however no answer given on Avvo is intended as legal advice or intended to create an attorney-client relationship. Dan's expertise lies in the electronic entertainment (video game) industry, as well as complex internet law issues, electronic free speech, entertainment law, copyright and trademark law, and computer fraud. He primarily represents game developers and founders of emergent internet technologies.
If "crowdfunding" sites are "taking off," I assure you that the SEC will be shooting those sites down very shortly.
New laws call for the implementation of regulations to facilitate crowdfunding, but there are none finalized at the moment. As I understand it, a self-regulatory organization is going to have to be appointed to have oversight of the crowdfunding arena. What that means is whatever you're thinking of doing, someone at FINRA is going to be looking at whatever it is you're planning to do. So if you're hell-bent on designing a website and want to have it up as soon as all of the regs and rules are in effect, work with a securities attorney who has experience in dealing with FINRA (who will likely be the SRO for this) or a broker/dealer consultant.
The foregoing is not legal advice nor is it in any manner whatsoever meant to create or impute an attorney/client relationship.