Once you have created a debug configuration, you can launch a debug session from the Debug dialog by clicking the "Debug" button. If you have an existing configuration you can re-apply it in various ways to launch the debugger. If you have only on such configuration you can simply press the Debug toolbar icon. Usually you will have more than one launch configuration so perhaps the easiest way is by clicking on the down-arrow next to the Debug button on the toolbar and selecting the desired configuration from the list.
If you are not in the Debug perspective Eclipse will ask whether you to want to switch to it. (Eclipse will remember your answer if you tell it to do so, and will not ask again.) Once in the Debug perspective you will see your application in the debugger.
The upper left pane contains the debugger controls and stacks view. At the upper right are the variables and breakpoints views, among others. The source files appear in the middle on the left, with the Outline view to the right. The debugger Console view is on the bottom.
The source view will initially show the source file containing the next line to execute, which in this case will be the elaboration of the main subprogram (if that is the entry point specified in the launch configuration). The green line and blue arrow highlight that line, as shown in the figure below.
You can find more information about the CDT debug facilities in the C/C++ Developer User Guide in the Eclipse help system.