The first step is to set a breakpoint on the line where you want your program to stop. Left click on the line number to the left of the line and you will see the first character of the line change to a bright red background. Left click again if this is the wrong line.
You can also right click in the line number column and popup a menu which you can use to set or drop breakpoints. This can be quite handy if you have set a lot of breakpoints and you want to drop them all.
After setting one or more breakpoints, you need to click on the Run
icon in the run toolbar.
The run icon is the green alien icon 
.
This will save your source code file, run the proper
compiler for your code and then start the gdb debugger on the
compiled program. When the program reaches a line with a
breakpoint, it will stop and ebe will highlight the line
using a pastel blue-green background. The
highlighted line will be the next line to execute.
Both the Next 
and Step
icons will step through your code, line by
line. The difference is that Next will stay in the current function or
subroutine, while Step will step into a function if one is called on the
highlighted line. You generally only want to use the Step button to
step into a function in the same source file.
The Continue icon 
will resume normal execution of the program
and it will only stop if it encounters a breakpoint. You probably
would use this to rapidly step past some debugged code to reach a
breakpoint in some code which currently has an error.
The Stop icon 
will end your debugging session.
You can define variables using either the name of a variable in the editor of using the address of a variable shown either in the data window or the register window. User-defined variables are displayed automatically in the data window each time control is returned to ebe from debugging.
To define a variable by name, you can mark the variable first and then use a right click to bring up a dialog to define a variable. In this dialog you can edit the format and the size of data items as needed. The first and last values are used as array indices for the variable being defined. If this is not an array, leave them both as 0. If this is an array, set the indices as you wish. Remember that the first index is 0.
A quicker way to define a variable by name is to simply use the Control key and a right click to have ebe mark the variable automatically and initiate the dialog.
To define a variable by address, you can mark the address in one of the value fields in the registers window or the data window and then use a right click to bring up a dialog to deinfe a variable with the selected address. The fastest way to define a variable by address is to use the Control key along with a right click.