U can book the very first appointment by calling in or walk in. Subsequently, u can use MiHealthcare app (most hospitals and institutions) or HealthBuddy (SingHealth) app to manage your appointments, eg. to change next appointment date and time.
Yes there is a online portal, singhealth. Depending on which poly it is under. Otherwise easiest way is to call them up instead of waiting at the clinic. It saves the trouble of queueing.
You can find more about polyclinic make appointment here : https://community.theasianparent.com/q/how-book-online-appointment-polyclinic-saturday-possible-only-mon-fri/21835
thanks