Confused with Enhancements, BADI's, User Exits, Customer Exit

Hello Everyone,

I am so confused with identifying the right kind of exit or enhancement technique for a particular task.

I want to start from the scratch, when we get a requirement for an enhancement for a particular transaction code, how do we identify if that transaction code modification can be achieved by BADI or implicit or explicit enhancement or Customer Exit.

Could we achieve all tasks of functionality in one enhancement technique.  I am not able to figure out how:

Example - for a transaction Me21N,  if  I get requirement of validating a user entry/adding a new field/screen/messages or for that matter adding any new function that is not incorporated in the standard functionality, how do I find out the right kind of enhancement technique - how do I know if new Enhancements Framework (Implicit/Explicit/BADI) or Customer Exit or User Exit or VOFM Routines or BTE fulfills the requirement.

I saw different posts on finding exits but still not able to figure how to find what kind of enhancement technique to use.

Need Help. Any inputs will be much appreciated.

Thanks.