I didn’t think I was going to write pilot tips anymore, but this one may be a good one to bring to Pilot’s attention, just as a little reminder for our pilots who are approaching an airport and are preparing for landing.
In short: how you report inboud with Tower, depends on how you approach the airport.
During your descent you will come to a point where you need to enter the airport’s Pattern.
Read more about pattern entry in the User Manual.
Radar Controller
If there is a Radar Controller present, he will guide you to the Pattern. Fly your flight plan until Approach guides you differently. Your flight plan is not holy; always be prepared to deviate from it, when instructed. At what point Approach hands you over to Tower depends on the approach type; a Visual Approach is different from ILS approach. And reporting inbound is also different from how you approach.
No radar controller
When Approach frequency is not staffed, you are responsible for your approach. You need to make your initial descent and approach and maintain aircraft separation. When your near the airport, you report inbound with the Tower controller. And Tower will instruct you how to enter the Pattern (via downwind, base or straight in).
Reporting inbound with Tower
If I would have a penny for each time Pilots report inbound incorrectly… 
Again: how you report inbound with Tower depends on how you approached the airport. It’s nice to get it right.
Check out the Landing and Communication tables in the user manual for more detailed info