I was in the same boat a month ago with some demijohns i went Through theses steps
Rinse with hot water and scrub with a bottle brush ( you can get them from home brew shops )
For more stubborn stains soaking in boiling water should shift it.
for the really tough stuff I was using diluted...
If its fizzing when you add sugar it sounds like its fermenting. Is this a kit ? Have you got it in a bucket with a air lock ? I use air bubbling through the airlock to tell me if it fermenting or not .
I'm making elderflower sparkling wine and I'm trying to work out what bottles to use I know the swing top glass bottles are good but I herd they sometimes explode, so I was thinking of using plastic ( PET ) screw top bottles instead ( less shrapnel ).
Any one have any knowledge of this ?