HN user
You're actually employee number 1 - as in theory, the co-founders are only taking home enough money to keep a roof over their heads.
You're also the first technical member, which means that you'll be building the entire product. You are responsible for the execution, which means you are critical to the success of the company.
Start negotiating at 20%.
The biggest thing you'll want is Value-Based-Fees.
I've written about them here: http://shipsolid.com/blog/why-value-based-quotes
You'll run into a lot of people that just want a website for their business/family/dog whatever - 5 pages or so. You can do that, but you'll just end up selling your time for cheap and racing to the bottom.
Don't do that. Instead, develop skills that demand a premium, and meet real business needs by creating money and value for your clients.