Sequencing is ATC's job. Once you have been cleared to join the circuit, ATC will sequence you with existing traffic. You may hear calls like "Number two, follow the Piper on base" or "You are number three in the circuit." This is crucial situational awareness. You now know your position in the landing queue.
If ATC says "Number two, Cherokee on base," you know there is one aircraft ahead of you in the circuit. You can plan your descent and timing accordingly. You must now look for that Cherokee and confirm you have visual contact.