Maintaining an old bike properly
If you must leave a bike outside during winter, the best thing to do is to shelter it as best you can (eg, under an eave, but not in the "dripline") and cover it with a sturdy plastic tarp.  The tarp should be tied down well to keep it from blowing off, but should be open at the bottom to allow air in.

Before storing the bike, squirt some chain oil on the chain and sprockets, and squirt some aerosol lube (not WD40 but maybe something like LPS or SuperLube) down the brake and shift cable housings.  Dab off excess but leave a good coating.

Ideally, hang the bike or store it upside down, so that the weight is not on the tires.  (Standard quality tires and tubes will hold enough air to support a bike without going flat for anywhere from about a month to 6 months, but will only hold enough air to support the rider for 2 days to two weeks.)

Otherwise, maintenance is based on wear.  Clean and relube the chain once a season or (ideally) every 500 miles, replace the chain after maybe 2000 (get yourself a chain stretch gauge to check this).  Have a bike shop check the sprockets for wear after maybe 5000 miles and then every 1000.  Have the wheel and headset bearings relubed every 10,000 miles or so.  Replace tires when the tread wears through or when the sidewalls develop cracks.  Replace tubes when you replace tires.