Crowdfunding, especially for a startup, is unique to each individual and company.
Most crowdfunding platforms, at least the more established and popular ones, have essentially become preordering platforms. Without the promise of a product of reward, you will find a hard time raising money in the viral fashion a lot of them want you to believe is a probable outcome.
The key is to be on a platform that best represents your company and has an audience of contributors who align with your current position and value proposal.
Kickstarter, Indiegogo, and Tilt open are good if you have a product you can distribute soon after raising your funds. They fall under the preordering type of platform.
If you are a startup working on your product (could be tangible, could not) and don't plan on having physical rewards and packages to offer contributors in a timely manner, those type of platforms will do you little good unless you have deep connections and a preexisting group of supporters. In fact, a lot of crowdfunding platforms won't even let you participate unless you can offer a product, service, or good in exchange for contributions.
Offering equity is an entirely different form of crowdfunding. You'll want to use a platform like fundable. Even angellist has features that resemble crowdfunfing, although not in the traditional sense. These platforms have an audience of contributors who are looking for investments, not just a way to preorder a product you may or may not be ready to deliver.
There will be costs associated with both types of platforms, whether it's a monthly fee or commission from the funds you raise. But ultimately from a crowdfunding site, you want to choose the values and mindset of the inherent contributing audience. And that will outweigh any cost of participation because it will ultimately determine your success.