EF (entity framework) usage of "using" statement

I have a project on MVC. We chose EF for our DB transactions. We created some managers for the BLL layer. I found a lot of examples, where "using" statement is used, i.e.

public Item GetItem(long itemId)
    {
        using (var db = new MyEntities())
        {
            return db.Items.Where(it => it.ItemId == itemId && !it.IsDeleted).FirstOrDefault();
        }
    }

Here we create a new instance of DBcontext MyEntities(). We using "using" in order to "ensure the correct use of IDisposable objects."

It's only one method in my manager. But I have more than ten of them. Every time I call any method from the manager I'll be using "using" statement and create another DBcontext in the memory. When will the garbage collector (GC) dispose them? Does anyone know?

But there is alternative usage of the manager methods. We create a global variable:

private readonly MyEntities db = new MyEntities();

and use DBcontext in every method without "using" statement. And method looks like this:

public Item GetItem(long itemId)
{
    return db.Items.Where(it => it.ItemId == itemId && !it.IsDeleted).FirstOrDefault();
}

Questions:

1. What is the proper way of using DBcontext variable?
2. What if we wouldn't use "usage" statement (because it affects the performance) - GC will do all for that?

I'm a "rookie" in EF usage and still haven't found the unequivocal answer for this question.