1990 Isuzu Trooper rough idle

90 isuzu trooper 4cyl 2.6L. 160,000. When I first start it up it sometimes acts like its going to stall or it does stall. Sometimes it doesnt idle rough. Everytime when it warms up slightly it acts like it stalls. After its completley warmed up it idles fine. Also I have white smoke coming out the exhaust. I dont know a lot about vehicles so I dont know if that has anything to do with the idling. I was told that smoke from exhaust is an internal engine problem. I just need some info before I go to a shop cause whatever the mechanics tells me I have to take his word because of my lack of knowledge. I just need to know what to expect.