Very little. Basically, you will need:
- Your custom URL redirecting to a landing page that explains the basic founding premise (or not) - there are many startups whose landing page simply says "coming soon".
- A Twitter handle
- A LinkedIn page
- A Facebook Page, if Facebook is relevant for your target market
You don't need to be an award winning web designer / developer to get this done. There are hosted services that can help you put up a gorgeous landing page in no time at all using a drag-drop interface. SquareSpace is one. Unbounce is another one which is really great if you want to do some quick landing page A-B tests.