Let's say `$website` is the instance of your website entity.  
You can get the store view ids for a website like this:  

    $storeIds = $website->getStoreIds();

This will return an array;  
If you want the first store id (the only one in your case) use `$storeIds[0]`.

**[Edit]**  
If you have the website id, you can get the website object like this:

    $website = Mage::getModel('core/website')->load($websiteId);
then use the code above.