so simply,
metal oxides: Form basic oxides. Lhs of periodic table.
Non metals: Amphoteric oxides near middle of table.
Non-metals: Basic oxides near rhs of table.
Nobel gases = no oxides.
The more metallic, more basic the oxides.
The reason non metals form acidic oxides is because, when the gases dissolve in water (or go into solution) they form acids. I'll give an example.
2co2 + h2o ----> 2hco3
hco3 <----> h+ and co3-
the presence of the h+ ions makes it acidic. (it's actually h3o+ but dnt worry about that)