i am currently creating an android app. I dont know if i should use already existing source codes for that particular app, then modify it and add my own innovations or if i should just create a new source code from scratch

please help

hi dear; I want to help but ur question is not properly structured. what are the features of ur prospective app? what existing source code are u contemplating?

this should determine whether to tweak...or build

There is nothing wrong using open source code to start up your own projects. For me, it usually boils down to specifics that other coders might find neccessary. For example:
1.

There is nothing wrong using open source code to start up your own projects. Be careful however, some of those source code might have been created for educational reason without putting due concern to productivity, you might still have to do some tweaking for stuffs such as error management, security. Asides that it usually just boils down to specifics, depending on the nature of the project and the specific code you want to use. Legal issues, design patterns, methodology of the code, Backward compatibility, Longevity of the code, efficiency......