Hello,
I am new to Enonic and I am wondering if there is some sort of ecommerce/webshop application available.
If there is one then I would be happy to get some information about it.
If there isn’t one then how should I start with creating a valid webshop in Enonic?
Thanks for any help given 
Yes, you can take a look at the PayUp! Store app: https://market.enonic.com/vendors/iskald/no.iskald.payup
I would like to point out that any real commmerce features like orders and carts should be stored using the node API rather than the content API. And any commerce management features should either be built using a custom admin tool or through the site itself. The payup store is extremely simple and was built before the node API was launched.
Can we manage objects created with Node API in Content Studio?
Or do we have to create a custom admin interface for that?
Not out-of-the-box. You could potentially by building a widget (a panel added to the right in the browse view), but I guess that would qualify as a custom interfance anyway. The simplest approach is probably building the management directly into your website, only available for “admins”.