How do I get a website id from a store id?

Like this:

Mage::getModel('core/store')->load($storeId)->getWebsiteId()

Or do a direct database lookup on the core_store table.

Just for completeness: From the current store you're in, you could retrieve the websiteId as follows:

Mage::app()->getStore()->getWebsiteId(); 

In Magento 2 to you can find the Website Id using store id like this:

$this->_storeManager->getStore($storeID)->getWebsiteId();