My 2000 Cadillac Seville STS

Hey, I own a 2000 cadillac seville sts and I have recently had troubles with it. Well I senn my car was leaking anti-freeze out of the overflow cap. So I went and bought a new cap and now that I put it on there, white smoke constantly rolls out of the exhaust and sometimes when I go to start my car it clicks and does nothing else but after a few times of turning it over it finally starts. But now when it starts up after it clicks a few times the engine starts shaking really bad. Also when I am driving down the road, lets say about 50 miles an hour the temperature gauge starts to rise but then when you get on it and speed up the temperature gauge goes back to normal. Do you have any suggestions about whats wrong with my vehicle?