I locked my keys in my 1999 chevy cavalier, I want to know if I can use a slim jim on it to unlock the doors

If you know what it is that you are trying to grab inside the door with a slim jim then 'yes' it is possible to open the car this way. but if you have never slim jim'ed this type of car before then it is best to have a locksmith come out and do it proper. there is a chance that you could knock off a linkage rod and make this situation even worse.
but if you want to give it a try then I will try my best to tell you what it is that you are looking for inside the door. near the back of the door where the door latch is located,, you are trying to grab onto the inside linkage control tab that is on the latch itself,,, with the slim jim you should feel a spongy feeling while pushing down,, at that point slide the slim jim in alittle and twist and then lift up --- this should unlock the car.
now there is another way of attacking this door,, but you will need to use a heavy wire bent like a 'L' and at the bottom of the 'L' you make a small hook to grab onto the inside control linkage rod ( inside the door ) --- you will need the heavy wire,a propane torch ( to bend the wire ) and two small wooden wedges to wedge the window away from the door skin. if you have your tool made ,, go about mid way to the front of the door and lower tool into the door about 7 inches and try and hook the linkage -- watch the inside lock control for movement while you fish for it,, grab it and move it backwards -- this will unlock the door.
'if you do try to slim jim the car - I would recommend trying the passenger side door so if anything does happen to one of the linkage rods, then it will not affect the drivers door.
I know that this all sounds kinda tuff and time consuming -- and it is,,, that is why I would still say to call a locksmith and have them come out to the car and pop it open for you,, they will be there in about 20 minutes -- it will take them 2 minutes to open the car and the cost will be about 45 to 55 dollars for this service. now you can also call a taxi service or a tow truck service and they might do it slightly cheaper then the locksmith company -- but they are far from qualified to do the job ( but they can get it open cheap -- ) -- I would stick with the skilled locksmith ...

c...