You have to register a My Account (fiddle around with top right LOGIN, and register)
To add services into your My Account, you then have to know your account number, which may not be written anywhere, so you have to go onto chat and ask. Chat will do it all for you anyway.
I would start by downloading the My Optus app. This shows usage, can be a little behind, and will also you your account number under billing.
If you have only just activated Optus tends to be a little show getting the information between their systems but the app will show you the details over the next few hours, hopefully.
If you have an Android or Windows phone you should set your own monthly warning and limit on the phone itself under SETTINGS.