Rough idle; engine shaking

2003 monte carlo 3.4 liter engine 94000 miles.
the car sounds like its struggling to stay on. I was told to replace the gas cap and give it some time. I replaced it one night at about 6 in the evening and at 10the next morning I started it again and the problem persisted. The engine shakes when I start it as runs as well. I would appreciate some help.