Required steps for investments? Can private investments be simply automated?

Suppose I want to set up an area on my startup's homepage for pretty much anyone to invest in my company - whether it's $5 or $5,000,000 - and in return they of course get a percentage of the company proportional to their investment. The process should be automated. How would I go about this?

I've done a bit of research on the matter but I must not be using the right keywords because I haven't found what I'm looking for. If there are already services out there like this, I'm not aware of them and would love to hear what you all have to say about them.