When flying into a towered airport, you will be given specific instructions, as per the ATC Handbook, Section 10, Arrival Procedures and Separation such as:
"make left traffic for runway 15"
"enter a right base for runway 15"
"make straight-in runway 15"
You can also request whatever you want, like: "Tower, Cessna 3-3-Lima, information bravo, requesting left base for 15"," and the controller will then evaluate your request and issue you instructions.
These instructions refer to the following diagram form the Pilots Handbook of Aeronautical Knowledge (PHAK, p. 13-12):
Traffic patterns are also covered in the Pilots Flying Handbook, Chapter 7., and states that:
When operating at an airport with an operating control tower, the pilot receives, by radio, a clearance to approach or depart, as well as pertinent information about the traffic pattern.
Which type of traffic pattern information ATC might give you, depends such factors as where you are relative to the airport, other traffic, noise abatement, obstructions, local procedure, etc. For example, in the diagram below:
- N or NE of the airport, you could be given "Left traffic"
- NW you could be given a base entry
- W you could be given a straight in.
- If you're SW, S, or SE of the airport, you could be given the same pattern instructions as the north side, but with RIGHT instead of LEFT, or you could be instructed to overfly the field and enter, say, left downwind.
If you are in contact with ATC, and don't know what you're expected to do, then key the mic and ask the controller.