High and Low Rev's at idle.

[Crazyric88]

Hey people, Im driving a BMW 318Ti petrol 1998 model. Ok so il give as much detail as i can but im not great with cars. My car has been off the road for abot a month now, i installed a new battery today as the old one was dead. The car started up fine but i noticed that my rev's were all over, the rev's were dropping so low that the car sounded on the edge of stalling, but i also noticed that once the engine got a little warmer the rev's would go to 1500 for a few seconds then drop to 800 then drop to the point of almost stalling. I checked my water and oil levels and all was fine, oil may need a little but nothing major I let the engine warm up but still no change. Alsoit maybe me being paranoid about my car but white smoke comes from my exhaust, not alot of it and its not dense but i can see it, but that maybe because of the cold weather. Last of all when i turn my light on in the car all my dashbored lights go dimmer, that has been happening since i got the car though. That is everything i can tell you at the moment, iv read things like cleaning filters and so on but i wante to get a response on here 1st.