Often on older bicycles the chrome is getting bad and brown rusty.
There are more possibilities to restore it to an acceptable level.
When the chrome is gone it is not possible to restore it but hiding is possible.
One of the best ways is to use Brillo steel scouring-pads with soap.
With a little water on the pad you can get the chrome to a real shinning state. 

When you rinse the chrome with hand-warm water there will be a film left and will protect the chrome longer.
Most of the time this is good enough for the chrome-work on a bicycle.
It is also possible to use aluminum foil and wrap that into a ball than use the ball to clean the chrome.
Turn the ball around and re-wrap it frequently so you will always have a fresh piece of foil.
The chrome will start to shine and cover the bad spots.
When the rust is hard and the chrome is completely gone it is no longer possible to restore, so don’t expect miracles.
The chrome quality on bikes is most of the time good and will stay for a long time in good condition. 
Re-chroming is very difficult with a bike frame, often only the rear end is chrome and that can’t be chromed again.
It is important not to leave any paint or dirt in a chroming process, or the process will not work.
Front-fork and loose parts can be chromed easily.
All the old chrome must be removed with a sand-blasting gun, and buffed until it shines like a mirror.
Every little scratch you leave behind will be like an existing scratch in your new chrome.
At last the parts to are being chromed must be cleaned very thorough before the chroming process can take place.