It'll be straightforward!
Call the number and normally a recorded message plays, asking you to enter your access code. You usually have to enter # after the access code (but may be another button, just listen to the message or follow the instructions you have).
Depending on the setup, you may go into the call directly or another recorded message plays, asking you to state your name followed by another # (I normally skip this part and press # directly).
And then you'll be in - either the person chairing the meeting will ask "who's just joined?" or you just state "hi - it's thyrilthneer". (If the chair hasn't opened the call yet, you'll hear hold music until they do, and then lots of beeps as everyone joins.)
If you mess up and enter the wrong access code, there will be a message saying your code isn't recognised. Either enter it again or you can just hang up and redial.
Hopefully this helps! You can probably practice this evening by just dialling the number and listening to the instructions if that will reduce your anxiety - unlikely another conference call is happening today, so you'll just get a message (after you enter the access code) saying that you're waiting for the meeting to start and the hold music.