I've created a custom list and added one column of type hyperlink.
How can i change the default value (http://) into another value? Is this possible?
Natively there isn't a way to use a default value with a hyperlink field. A singline line of text coulmn might work, but depending on the URL you might hit the 255 character limit. You could use some javascript/jQuery to set a URL and description by default.
You can use Single Line of text and have the Default value what you need it. You would be losing the formatting feature though.
You can also probably try to create a custom column type
You can try Eventhandlers ItemAdding/ItemUpdating for changing the default values.