To Get Mew
First you'll need to get all of the signs in the levels. Then Oak will tell you about the Rainbow Cloud. You can go there and that's where you will find Mew. You'll see him in a bubble. Throw pester balls at the bubble and when it breaks Mew will fly away. Look to the left and you should see Mew trying to get back in its bubble. Quickly take a picture of it.