Ok so you need to first deliver the Ruby and Sapphire to Celio at the One Island pokemon center before you can trade with Ruby version. The Ruby is located inside Mt Ember's secret entrance that the Team Rocket Grunts open for you after you receive the National Pokedex upgrade from Professor Oak.
You can get the upgrade by registering 60 pokemon in your pokedex (they must be caught or obtained) and then speak to Professor Oak. If have not been registered in the Hall of Fame at least once yet you must do this first by defeating the Elite Four and Champion at the Indigo Plateau. Once you have the National Dex go to Mt Ember on One Island and locate the Rocket Grunts to the right and battle them and win.
Go inside the secret entrance and make sure to bring a pokemon that knows the move Strength. Use Strength to move the boulders until you reach an intersection where you can go down or right. Choose down and continue using Strength to reach a room where the Ruby is located in. Take the Ruby and go take it to Celio at the One Island Pokemon Center (he's the guy by the big machine wearing glasses).
He will reward you with the Rainbow Pass which allows you to reach islands 4-7. Go to Four Island and head to the right and use Surf and go acrossed the water to reach Icefall Cave. In here step on the ice carefully to reach the item HM08 which contains Waterfall. Teach Waterfall and use it to climb the waterfall located in the first room of the cave.
Go to the ladder and you should find Lorelei of the Elite Four and some Rocket Grunts. Take care of the rockets and speak to Lorelei. Now go to Six Island and head southwest until you reach a cave with a door on it. Use Cut on the door to open it and go inside. Fall down the hole in the first room and you will appear in a room with four holes. The trick to this cave is that you must fall down the holes in this order: Up, Left, Right, Down.
Doing this will send you to the room with the Sapphire in it. Take the Sapphire but a scientist will steal it away from you. Head to Five Island and go right until you reach a warehouse. Check the door with A to unlock it and you will be inside the Rocket Warehouse. Carefully use the arrow panels to maneuver yourself to the top right of the warehouse and you will find the scientist who took the Sapphire.
Battle him and win and he will give it back. Head back to Celio at the One Island Pokemon Center and deliver the Sapphire and he will activate the machine. This machine allows trading between Ruby, Sapphire, Emerald and even Pokemon Colosseum and Pokemon XD Gale of Darkness.
NOTE: If have not rescued Lostelle yet on Three Island you cannot go inside the secret entrance of Mt Ember. Go to Two Island and go inside the Game Corner and the owner will ask you to save his daughter. Go to Three Island and go northwest until you reach a forest. Go inside to reach Berry Forest. Inside here you must continue going northwest until you reach the lost girl. The lost girl will be attacked by a Hypno when you speak to her, battle the Hypno and win. She will reward you and now you can go back to the Game Corner on Two Island and talk to her dad. Her dad will be pleased and you will also deliver the Meteorite to him at this time.
If you mean another trainers Pokemon you have to use cheat codes
you can't you can, but only by trading all your Pokemon to another game before restarting, then trading them back after you have gained the ability to.
You can get more than one starter Pokemon by trading with another person.
Not without AR or trading it from another game.
you either have to trade with a friend or use an action replay code to catch another trainers Pokemon.
you dont
Action Replay
Simply defeat another Pokemon either from the wild or a trainers Pokemon.
If you haven't done all the side missions on the Sevii islands dealing with Celio, then you won't be able to trade between regions
If you mean another trainers Pokemon you have to use cheat codes
by trading
You can't. You only get one without trading it from another Pokemon game.
NO
to catch another trainers Pokemon on pearl you have to have an action replay system or some kind of flash card
Not without trading from another game.
you can't you can, but only by trading all your Pokemon to another game before restarting, then trading them back after you have gained the ability to.
You cant.