I use the Cokin system for lens filters. I have a tungsten colour correction (A80) filter, an orange filter, and a linear polarisation filter. Of these, I found the polarisation filter the most useful. I found the quality of the colour filters a little suspect (but haven't checked this in detail), but the pola filter looks fine.
I bought all the stuff second-hand, and I found it's a very inexpensive way to get going in serious photography. Now, on to some details...
When you take off the bottom of the camera, you can see three pot meters. One is for the low light level/slow shutter time end, one for the high light level/fast shutter time end, and one seems to be a sort of `bias' which has a small but noticable effect on all light levels. (I will produce a diagram of the positions of the pot meters when I find the time!)
The camera can be calibrated with help of a camera with correctly-functioning TTL meter. Find a plain surface to measure light. Set the shutter of the GOOD camera to 1/500 or 1/1000s and set the aperture so that the light meter says the lighting is correct. Now set the BAD camera to the same setting, and tweak the high-end pot meter till the light meter shows the same reading as the good camera. Now do the same with a slow shutter speed (1/4 or 1/8s or so), but tweak using the low-end pot meter. When you go back to the 1/500 or 1/1000 situation, you find that the bad camera now gives a wrong reading, but probably less wrong than before. When you repeat this procedure, it's likely you'll eventually converge to a good calibration. You can check your final result by comparing the meter readings at all different shutter speeds.