how to implement UnitOfWork/transaction in C#

I am trying to implement UOW with Generic Repository pattern. Here is how my src looks like

public interface IRepository<T>
    : IDisposable where T : class
{
    void Add(T entity);

    void Delete(T entity);

    void Save();

    void Update(T entity);

    IQueryable<T> GetQuery();

    IEnumerable<T> GetAll();

    IEnumerable<T> Find(Expression<Func<T, bool>> predicate);

    T Single(Expression<Func<T, bool>> predicate);
}


 public interface IUnitOfWork
{
    void InitTransaction();

    void Rollback();

    void CommitTransaction();
}


public class EntityRepository<C, T> : BaseRepository<C, T>, IUnitOfWork
    where C : CustomerContext
    where T : class
{

    protected DbSet<T> DataSet
    {
        get
        {
            return _ctx.Set<T>();
        }
    }

    public EntityRepository(C ctx)
        : base(ctx)
    {
    }

    public override IQueryable<T> GetQuery()
    {
        return DataSet;
    }

    public override void Add(T entity)
    {
        DataSet.Add(entity);
    }

    public override void Save()
    {
        _ctx.SaveChanges();
    }

    public override void Delete(T entity)
    {
        DataSet.Remove(entity);
    }

    public override IEnumerable<T> GetAll()
    {
        return DataSet.ToList<T>();
    }

    public override IEnumerable<T> Find(Expression<Func<T, bool>> predicate)
    {
        return GetQuery().Where(predicate).ToList<T>();
    }

    public override T Single(Expression<Func<T, bool>> predicate)
    {
        return GetQuery().Where(predicate).SingleOrDefault();
    }

    public override void Update(T entity)
    {
        _ctx.Entry<T>(entity).State = System.Data.Entity.EntityState.Modified;
    }

    public void InitTransaction()
    {
        throw new NotImplementedException();
    }

    public void Rollback()
    {
        throw new NotImplementedException();
    }

    public void CommitTransaction()
    {
        throw new NotImplementedException();
    }
  }}

This is where I have implemented IUnitOfWork in DbContext class.

public class CustomerContext :
    DbContext, IUnitOfWork
{
    private DbContextTransaction _dbTransaction;
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
    }

    public void InitTransaction()
    {
        _dbTransaction = this.Database.BeginTransaction();
    }

    public void Rollback()
    {
        _dbTransaction.Rollback();
    }

    public void CommitTransaction()
    {
        _dbTransaction.Commit();
    }

    #region DbSet

    public DbSet<User> Users{ get; set; }
    public DbSet<Contact> Contacts {get; set;}

    #endregion DbSet
}

EF Relation

One to many -> One user can have multiple contacts

Referred to number of articles but not sure how do I put the User & Contact CRUD in one transaction.

public class CustomerController : APIController
{

 }