Can someone help me on Pokemon Emerald?

To get Deoxys you have to go in the rockit. To do this takes time so just keep doing the Pokemon leage.

Get an Action Replay Device if you haven't already, and look on the Supercheats Emerald Action Replay code pages for the Old Sea Map in your PC and Mystic Ticket. Or just use the warp codes for Faraway Island and Navel Rock. Deoxys is on Birth Island, to get there get an Aurora Ticket or warp to Birth Island. Oh, and ignore the warp codes which say 'put this code in before the regular warp code yada yada...' because i've tried it and it DOESN'T work. Well, don't ignore the codes, but don't put the code in before the warp code. Turn your AR off when you get there, though, otherwise you'll never stop warping to that place. Once your business is done, simply go and talk to the ship guy and he'll say "yada yada would you like to return to lilycove?" Say yes.

MEW RULES!
Psymaster :-)

You need to use a cheat device or attend a Nintendo Event.

You have to go to a special nintendo event in america to get these following pokemon: latias or latios (depends if what colour you pick) trade records with ruby or sapphire then you get the EON TICKET. It is at lvl 50, deoxys you need the AURORA TICKET it is at lvl 30 after doing a puzzle,
For Mew you need the OLD SEA CHART and it will play hide and seek until you battle it. It's at lvl 30. Lugia and ho-oh. You need the MYSTIC TICKET. Once you arrive you will be able to take 2 paths. One leads to Lugia and the other leads to ho-oh. They are both at lvl 70 and use the itemfinder where ho-oh was to get the sacred ash AND ho-oh can also be earned if you have a gamecube and just a Pokemon colloseum with no bonus disk after purifying all 48 shadows and complete colloseum mt battle 100 with no game overs to get the legendary Pokemon ho-oh in your PC. Jirachi you don't need to have a ticket you need a gamecube and pre-order Pokemon colloseum to get the bonus disk after purifying all 48 Pokemon or you can obtain it from Pokemon channel. And last but not least the rarest Pokemon out of the 386 celebi. You may be wondering how to get it but you cannot find it outside of japan.if you live in japan your in luck because it's the excact way to get jirachi except you celebi instead. So I hope that answers your question.sorry that the answer is so big