Nothing much to worry when u see a cardiologist for the first time.
First, expect that a nurse will first check ur vital signs (blood pressure,pulse rate,respiratory rate,temperature). Your weight & height may be measured as well if u do not have the recent record of them.
The doctor then would ask u details on ur health history, family history, some personal information,etc. Infomations that will be asked will help to get some clue of ur condition.
Then the doctor will do assessments so expect that u will be examined. An ECG may be done once more.
Expect also that a doctor will require some blood investigations. It will be required to be done at once if seem to be very necessary but if not really urgent, the doctor will require the results to be seen on ur next appointment.
Everything will depend on the initial assessments of the doctor. The following will probably be done as well but not necessarily all at once during ur first appointment: a chest X-ray, an ECG stress test or a 24 hour ECG monitoring, or probably a cardiac echocardiogram.
Just be relaxed - that will help ur doctor to find out what is going.