Your first opportunity will be in the Viridian Forest, it is rare, but you will eventually find a Pikachu in there. If you have gotten further, I suggest going to the Powerplant. If you travel towards the entrance to the Rock Tunnel, you will see a river going south along the far eastern part of land. If you follow it down you will find the Powerplant. Here you can find Pikachu, Magnemite, Voltorb, Electabuzz (Rare, maybe onlt in blue, but I don't remember :P) and if you can find it, the legendary bird of thunder Zapdos (level 50!) You might also encounter Magneton and Electrode, but I'm not 100 % sure about that.
A Weedle.
You can only catch them in Pokemon fire red.
You cnnot catch it in Pokemon fire red ,but you can catch it in Pokemon Diamond and Pearl.If you want it to be in your fire red,you will have to migrate it from your Diamond/pearl to your fire red.
You cant catch any trainers Pokemon
The old man shows you how to catch pokemon.
Unfortunately, you can't catch a Red Gyardos is Pokemon diamond, but you can catch one in Pokemon Heart Gold or Soul Silver. It'll be at Lake of Rage.
You cannot catch one. You can only choose it as a starter Pokemon in Pokemon Red and Blue.
You cant
The old man is not a Pokemon
you can get a mareep under viloet city
You can't catch it on Pokemon Fire Red. You have to trade it from Leaf Green.
In Pokemon Crystal version, you catch it at the Lake of Rage.