Ok so this all seems complicated but it just takes time.
Inbound
Check VTC for an inbound reporting point for the zone, usually a purple diamond. Im going to use my home airport (YBSU) as an example.
"Sunshine Coast Tower, CALLSIGN Inbound Details."
"CALLSIGN, Sunshine Coast Tower"
"CALLSIGN is overhead (VFR reporting point), (altitude), (atis receipt), (any additional information, over water, over land etc.), requesting inbound clearance."
Here you will usually get one of four replies.
CALLSIGN, make (circuit leg) for RWxx. Report (circuit leg).
Ok so we are cleared to operate into the Class D at our assigned altitude, and we are able to descend to circuit height if able. Once you report on that leg, they will generally issue landing clearance. Remember, you are cleared to descend to circuit height as soon as you are in the zone.
CALLSIGN, parachuting ops 2nm north of your position, do not penetrate Class D zone, retry in 10 minutes.
Even though they have acknowledged your callsign, you are NOT cleared into the Class D. Follow the instruction and call again.
Aircraft calling, heavy traffic at present time, try again at (Time xx)
Once again, not able to fly into Class D. Try again at the given time.
CALLSIGN, roger.
Cleared to enter Class D at altitude specified in inbound call. Must stay in current track until cleared otherwise. Wait for clearance for circuit direction or landing clearance. Eg, if I reported inbound at 1500 over water, I must stay with that track and altitude.
The important thing to remember is to listen, and never read back an instruction you cannot comply with or don't fully understand. Thats the only way you can get into trouble generally.
I'm only a 15y/o student pilot so if someone spots a mistake, please say so.