Deep,
Most importantly when an error/warning message is prompted functional guys need to know why the error is coming ?
Ans:: Go to debugging mode using /H tcode and Press F9, you will see a Pop-up there in ABAP commands type MESSAGE and click continue and then press F8 and go on with your processing. This will create a break-point where the message is being prompted from, you can know the reason why the message is being prompted here.
Using break-point you can go to a particular line of program and you may check the variable/table values there.
Using Watch-point you will know if the value of table/variable is changed.
F5 - step by step execution of code.
F6 - Executing a block of code at a time.
F7 - Execute the routine/FM at a time when you are in it and it can work like F6.
F8 - Total code is executed.