Implementing Generic Service for CRUD operations using EF6

I've done multiple projects using the Repository pattern(Interface=>Repository)but now, I'm facing a new challenge(not a big one) with one of my coworker's code. We are implementing a Generic service that contains all of the CRUD operations methods for all entity classes.

We are using Database first in this project, and have the following structure,

aspx.cs > entityclass(AppUser) > Generic Service > Entity model.

   public class UserServices : GenericServices<User>
   { //Implemented methods }

And this as the Generic Service:

public class GenericServices<T> where T : class
{
    App_dbEntities _db;
    IDbSet<T> ent;

    public GenericServices()
    {
        _db = new App_dbEntities();
        ent = _db.Set<T>();
    }

    public IEnumerable<T> Select()
    {
        return ent;
    }

    public T Select(string id)
    {
        ??
    }
}

I'm trying to handle the entities's properties but since this is Generic, it doesn't know what entity am handling at the moment. I've seen some examples using predicate as a function parameter. Please give me a hand.