If you use carbon, always use a good one like Rox or Fluval, and rinse it prior to putting it in the tank.
As for amount, I use 2-3 TBSP in my 100L system normally, if for any reason the water is looking cloudy or I see any problems I'll chuck in an extra 1-2 TBSP. I change it fortnightly.
The amount above is based on the recommendation which came with Rowa's carbon not fluval (who helpfully don't specify the amount), but i think it's close enough for mostly any quality carbon. Rowa say 500ml/1000L which is equivalent to 3 TBSP/100L.