run4yourlives raises some good points. But if you're sure you want to do the deal, the main thing you have to figure out is how much of the company your new in

run4yourlives raises some good points. But if you're sure you want to do the deal, the main thing you have to figure out is how much of the company your new investor will own, and how much of it will be owned by you and your partners. If you're not already incorporated you'll need to do that, and you can probably just include your investor as one of the founding shareholders (perhaps the only one who doesn't need stock vesting). If you are already incorporated, you can start with an angel financing doc from YC, and run it by a lawyer to make sure your case isn't special for some reason. If you're not sure where to find a lawyer, you should ask some startup founders you know personally -- this probably isn't a complicated scenario, but there's nothing like incompetent legal help to make a simple scenario into a gridlock.