After beating the Elite Four in "Pokemon FireRed," you will be rewarded with a whole new area full of different Pokemon. You will also be able to acquire the National Pokedex, which is capable of storing the information of every available Pokemon in "FireRed." Although the game is a remake of the original Game Boy version, it has extra features and extra Pokemon that would have been unavailable in the original games. To effectively "catch 'em all" in "FireRed," you will need to trade from other versions, not only Pokemon "LeafGreen" but also the "Ruby," "Sapphire" and "Emerald" versions.
Visit Professor Oak after you have defeated the Elite Four. You will find him in his lab in Pallet Town. He will replace your Regional Pokedex with the National Pokedex. The new Pokedex will hold information for many more Pokemon. Many of these can be found in the newly opened region, the Sevii Islands.
Return to Cinnabar Island, to the south of Pallet Town. You will find a ferry docked beside the island. This will take you to the first of the Sevii Islands, Knot Island. Here you will be able to trade Pokemon from the "Ruby," "Sapphire" and "Emerald" games. However, upon arriving you will find that the machine isn't working and it is up to you to find the Ruby plate to get it up and running.
Head to the mountain base on Knot Island and enter the cave. Navigate your way through the cave catching any Pokemon you don't already have along the way. Defeat the trainers inside and push the rocks out of the way and you will find the Ruby item. Leave the cave and take the item to the Pokemon Center. The machine will now be working and you will be able to trade with the other Pokemon games.
Explore the other areas of the Sevii Islands. You will find Pokemon from the Johto region in the wild, which have previously been impossible to get. It may take some time and effort but to fill the Pokedex, you will need to catch all these Pokemon too. Within these islands you will also find many of the legendary Pokemon from Kanto, Johto and Hoenn. Stock up on plenty of Ultra Balls and bring your strongest Pokemon in to catch them.
Trade with as many other versions as you can. To acquire every available Pokemon you will need to trade with "Pokemon Ruby," "Sapphire" and "LeafGreen" and to travel and explore every part of the Kanto region in "FireRed." Use every method and maintain your focus. There are many Pokemon to catch but with every part of the Sevii Islands unlocked and with the ability to trade with other versions it is entirely possible to complete the National Pokedex in "FireRed."