first you need to beat the goldenrod city gym. then you talk to a lady watering her plants in her house and she gives you a squirt bottle. after that, head north along the path through the national park and continue north and east. use the squirt bottle on the tree and it will attack you, fight and beat or catch it and you can go to ecruteak.
Go to Mt.Cornet and go to the lake where the it is nere the entrance to route 216 but it will take time.
u have to catch palkia,dialga,azelf,mesprit, and uxie u also need palkia and dialgas orbs.then you go to the top of monet corunet .arceus will apear and talk to u and you can do a misson for him when u come back he will let you capcher him. you can also get the asora flute
get a master ball then buy a max repel then go to the sea foam islands and use dig but as soon as you use dig turn off the power and you will have mew!! i tried this IT REALLY WORKS