Someone wants to invest in my startup. Now what?

I work as a network engineer but have been working on a project for the better part of this year. I showed the demo to my employers, with whom I am very close, and they loved it enough that they want to invest. We agreed on a number for what my partner and I need to operate for about 6 months, they told me a percentage of the business they want... but now what?

I haven't formed my business entity yet and that is first priority, from what I've read. I know there innumerable blogs and articles written about starting a startup, but can anyone give me any tips that I might not pickup quickly on my own or anything I might overlook?