So where did you buy the machine?
Was it new or used?
If it came with Lion and you bought it directly from Apple, you'd get the update to Mountain Lion free; if you bought it elsewhere, I don't believe it qualifies:
If yours does not qualify, you can purchase it at the app store.
PS: Technically, Mountain lion is not an update (free); it is an upgrade ($).
So what do i do about the fact that i cant install any apps at all ? And it was bought new from a store in denmark called Datagaarden, but i dont know where they got it from so that might be the problem. and yes it came with Lion.
Is this store an authorized reseller? If they're close enough, I'd stop back and ask (but don't get your hopes up).
ML is only $20, so I'd buy it if I wanted it.
As for not being able to install apps - can you explain the process/error message?
What apps exactly? Are they new apps you want or are they updates to existing apps?
Do they not install because they require Mountain Lion or some other reason?