A good way to find active freq's is to do a search mode in your scanner. Most scanners have pre-programmed service bands in them. Play around with that for a while, and write down the channels that you hear most often. Then look up the freq's on the FCC database, or here on this site, or even through various books published by myraid companies. Another way to search is to scan a particular category (aircraft, marine, railroad, public safety, news media, etc.) They fall under various bands like VHF Low, VHF Hi, UHF, 800 Mhz, 900 MHz, etc. This is another way to find active stuff around you. Again, as previous posters have stated: with such a broad general question, it's like finding a needle in a haystack.