Depends what kind of pokemon. Most evolve by leveling up, some using stones (firestone, moonstone, etc) some by trading, and some by trading holding a specific item.
In Pokemon Yellow Onix does not evolve, it is a one stage Pokemon.
you can evolve balbasaur to venasaur
Spearow evolves at level 20 in Pokemon Yellow.
fearow
Ive hear my friends say that in Pokemon yellow you can evolve eevee in a espeon with a moon stone
A yellow Floatzel of course
Caterpie can evolve into metapod starting at level 7. The evolution conditions for caterpie are not different in Pokemon Yellow from other pokemon games.
Moonstone makes pokemon evolve.
You'll have to trade your Kadabra in order for it to evolve.
It should evolve at Level 55.
16
Mew does NOT evolve