proper fit of a suit help

[nyarkies]

Hi! I'm new to this forum and I'm in need of some advice. I've used the search function and haven't found what exactly what I'm looking for.

I just bought a slim fit one button suit from Armani. My question is how should it fit properly in general? Is it acceptable to have a crease on the back in between the shoulders near the neck. How should the fit on the shoulders look? Any other things I should look at? If there is a guide out there, please post the link. Any help will be appreciated. Thanks.