If you want to do maintenance for your TGB 303R 125 then it is usually a good plan to have a Workshop Manual to use as reference in the event that you need to know techniques to strip off a certain part for example the Brake Discs and fit it again in the right way.
TGB 303R 125 Workshop Manuals are able to offer precise diagnostic info for the automobile therefore if something is wrong it helps you understand what the issue is and methods to correct it. You can then determine if this can be achieved on your own or that the job is simply too big you could make a booking for your 303R 125 in with a local friendly mechanic.
TGB 303R 125 manual