1998 Mercury Sable weird idling problem

So I went out to start my car and it took a long time to turn over. Actually, I had to give it gas to get it on. Once on, it idled at about 1500 rpms, and then when I shifted to drive, it died. I tried over and over again in each gear and ran it in park at 3000 rpms for a bit to test the idle, and it goes right back down, and dies when put in drive. Of late, the idle has been going up and down while on the free way, and when I come to stop lights and signs it goes really low and almost dies. This last week it has been sluggish, like it just cannot get enough gas. I do not know much about cars, so I did not think anything of these things. My bad. What do you guys think?