I have noticed alot of wild poppies lately in my garden, I left them as some of them are quite nice colours but now they are starting to take over, how have they become to grow here and how do I stop them ?

Since they're annuals, you can stop them from coming back next year by cutting the flowers. No seeds = no spreading.

I don't know what are wild poppies. Could you tell me more about this one? Where do they usually grow? Are the wild poppies harmful to our plants?

You can uproot them so that they do not spread. And just like crazy talk said, get rid of the flowers before they mature so that no seeds will germinate into new plants.