Regarding traffic, it depends on where your target market its, what resources you have already with your personal accounts, and what you're most likely to implement on a consistent basis with your limited time and resources.

In terms of "sign up to your site" I'm guessing you mean sign-up for your email list. Again this depends on your audience, but it can be something simple, like a video or pdf as long as it provides value and your message touches your target on an emotional level. This may take some testing a tweaking and you won't know until you get enough eyes on your opt-in to see if it's working.

You said your offering a service that is free, but it's also a business. Money has to come into play somewhere. To get people talking, help them with their problem. Send me a message and I can send you a link to a service that runs $1 a day that may be of help.