EF code first for existing db and model class

Question

when we work with EF code first with existing db then we have to write c# class which represent db table. so tell me when we write class then do i need to mention fields as PK or FK when table already exist ?

public class FooState
{
    [Required]
    [Key]
    [ForeignKey("Foo")]
    public int FooStateId { get; set; }

    [Required]
    public int State { get; set; }

    public Foo Foo { get; set; }
}