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
- An Instagram account, if IG is relevant for your target market
- A Snapchat account, if Snapchat 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.
Weebly, SquareSpace, Wix are all hosted platforms that can help you stand up a company website in no time.
For the more adventurous you can always use Wordpress which has many themes that include pagebuilder plugins (although Wordpress is significantly more difficult to use than the three platforms mentioned above.
See this other article for more pro-tips on standing up a website without writing code.