Hi Lubna,
Towards Indiranagar, RR Cycles in Ulsoor comes to mind.. not sure of their quality of service though as I have never tried it there.
But I think 90% of the stuff that is needed to keep a bike in good running conditions is pretty basic - you can read up on it, watch videos on youtube and perform the tasks with a minimal set of tools. What really drives me nuts is that most LBSs take longer to service a bike than most garages take to service a car! Learning to do your own basic service also has the added advantage that you are better prepared for any failures on long rides.
In terms of frequency of service - the most important part is the drive train. Keep a close look for dirt build-up and chain going dry. Depending on what conditions you ride in, you might need to clean and lubricate once in a few weeks to months.
Gears / brakes / cables would need adjusting depending on your usage again rather than the time intervals. Maybe once every month or so if you ride regularly.
The key is to inspect the bike regularly - weekly if you are commuting and a thorough once-over before every long ride.
Cheers!
Narsi