> What specifications must I use when creating and designing a poster using Photoshop?

What specifications must I use when creating and designing a poster using Photoshop?

Posted at: 2014-12-18 
So.....

I already do know what design to do for the poster, as well as the poster dimensions/size (I'm creating this for someone, and they already told me what size they wanted).

Since this is going to be printed, is there any other specifications that I must use/follow so that the poster wouldn't look bad/cropped when it's printed?

They told me that they'll be going to an actual printing store for the poster.

Do I use a 300px resolution?

Should I use margins and how wide should they be?

Is .jpeg the best file type to save it as?

Thank you!