What you do is trade pikachu with a red or blue game then once he is traded evolve him with a thunderstone than trade him back. But be warned once you do that you can't walk with him or talk to him. And he can't learn new moves. It baically runes the whole story line so I wouldn't bother evoving him at all.
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If this is the case, you're playing Pokemon Yellow. Yellow prevents the starter Pikachu from evolving due to following the anime, where Pikachu refuses to evolve (though it's possible to evolve it by trading it to another game, evolving it there, and trading it back to Yellow).
You cannot evolve Pikachu in Pokemon Yellow, unless you trade it to a red or blue version, and give it a thunderstone there. Once you get it back it will not follow you.
Give Pikachu a Thunder stone and it will evolve into a Riachu. But in Pokemon Yellow, the Pikachu will not evolve. You can get thunder stones easy by mining them in Pokemon diamond and pearl but in soul silver and heart gold you need to buy them with athleet points at the athleet shop
Evolving PikachuPickachu will not evolve in Yellow; it will refuse the Thunderstone, but if you trade your Pikachu from Blue or Red, you can evolve it with a Thunderstone then.
Pikachu do not evolve by level. They evolve when a thunderstone is used on them.