On a website, one can use a social sharing javascript library like addthis in order to propose share buttons to the user without having to program everything from scratch.>
On Android we have Intents for this. If you like to give the user an opportunity to share something, you can fire up an intent like this for example:
Intent share = new Intent(Intent.ACTION_SEND);
share.setType("image/jpeg") // might be text, sound, whatever
share.putExtra(Intent.EXTRA_STREAM, pathToPicture);
startActivity(Intent.createChooser(share, "share"));