How to unit test a component that calls observable service

I am trying to unit test a function that is subscribed to an observable service. Not sure where to start.

Component function I am trying to unit test:

  register() {
    this._registrationService.registerUser(this.form.value)
        .subscribe(data => {
          if (data) {
            this.errorMessage = '';
            this.successMessage = 'Account successfully created';
          } else {
            this.errorMessage = 'Error';
            this.successMessage = '';
          }
        },
        error => {
          this.errorMessage = error;
          this.successMessage = '';
        });
  }

Service:

  registerUser(user) {
    const registerUrl = this.apiUrl;

    return this._http.post(registerUrl, JSON.stringify(user), { headers: this.apiHeaders })
      .map(res => res.json())
      .catch(this._handleError);
  }