You can't, you'll can either trade from a friend who has one or catch it on Pokemon FireRed or LeafGreen and trade it to Ruby. The only other way is to cheat using Action Replay or GameShark.
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the mew code for ruby is 71E09BD117FDFD02 and mewtwo is 30621E6F164FED85 these are the right once i think
Only if you have a code breaker and the master code for ruby plus the mew and mewtwo code
the most powerfull Pokemon is mewtwo but people think its areceus when its not on Pokemon explores of time it is darkrai on darkness it is mewtwo on platinum it is mewtwo on dimond it is mewtwo on pearl it is mewtwo on blue rescue team it is mewtwo on red rescue team it is mewtwo on chores black it is mewtwo on ruby it is once again mewtwo i could go on for ever but i am trieng to make my point :)
ANSWER 1:Mewtwo appears only in Fire Red and Leaf Green. You can catch him in either of those games and then trade him to Pokemon Ruby.sorry mate its impossible only way is to trade like this guy said (: (: (: (: (: (: (:ANSWER 2:I regret to tell you that you cannot catch Mewtwo in Ruby. You can only catch Mewtwo in FireRed or LeafGreen. (You can use cheats to get it but it wouldn't be as fun).Mewtwo is at Level 70 in LeafGreen and FireRed so use an Ultra Ball/Timer Ball/Master Ball.If you need/want a Mewtwo you can trade Mewtwo into your Ruby/Sapphire/Emerald Game.
Go to Cerulean Cave. To get there, you must get the ruby and the sapphire and get the national dex.