White exhaust smoke

White smoke indicates water going through your engine or condensation in your exhaust. If it is a lot of smoke you probably have a small leak in your head gasket that seals back up as the engine warms and the materials expand. It will get worse as time goes on. If it's just a little smoke that doesn't last long it's probably condensation in your exhaust, especially if it's cold outside. Condensation is caused by the air cooling down in your exhaust after you shut down the engine.

You have a leaking head gasket, there are several additives you might try to seal it, but be prepared for lots of white smoke and an overheated motor if you leave it as is.

Sounds like a cracked cylinder head or head gasket problem

White smoke is most likely a blown head gasket or leaking valve seals allowing oil to leak into the cylinders

Valve seal leaks will cause an engine to smoke on start until the oil burns off. If a head gasket were leaking it would be smoking constantly.

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I concur with emissionswiz; it sounds like a typical sign of a leaking of 'blown' head gasket and the white smoke could well be coolant+water.

Rich fuel=black smoke.
Oil burning=blue smoke
Coolant burning=white smoke

Is the car runnig little rough on cold start up in themorning for few seconds?? Because I have done quite a few headgaskets on QG18 Sentra aka 1.8 liter nissan sentra. here is the test. Fill up your overflow bottle to the full mark and check the coolant level in about 5 days.(drive it normally as if nothing is going on) If the level is down, your head gasket is gone. How about the check engine light , is it on?? post results.