Hello I've bought MacBook Air for school this year and I was thinking about upgrading storage from 128gb to 512gb. I was thinking about Samsung 960 EVO 500gb. Will I need any adapter or will this ssd fit just right? Also, are there any cheap ssd enclosures so I can keep using my 128gb ssd for external storage?
No you cannot use that SSD. These MacBooks use proprietary SSD's and I only know of one company that provides them after market: OWC. You can check their website for upgrade info. They may have an enclosure that will work.
But to be honest, it will be expensive even if available and you are probably better off investing in external storage.