IT may help if you indicated what system you are specifically interested in listening to.
Generally speaking, if they are on a P25 system, and the system is NOT encrypted, you can program a digital trunking scanner to monitor this system. You will need a digital trunking scanner, specifically.
I personally have, and recommend, the GRE PSR500 handheld, or its "base station cousin". Many folks like the GREs, many folks are uniden proponents. The specific scanner manufacturer forums will help you make up your mind there.
Either way, you'll be spending a "not insignificant" amount of ca$h for a digital trunking scanner (plan on $350-$450 depending on what you want, and who you decide to purchase from).
With a subscription here (to have access to, and the ability to, download directly out of the databases here) and any one of several excellent programming software products available (I use and recommend Win500, though there are other excellent ones as well), programming these things almost becomes an afterthought, OR, at the very least, you can focus on how you want to set it up, exactly, without having to worry about the simultaneous issues of programming tasks.
Now, all of that being said (w.o knowing what system you are interested in specifically).....If it is an encrypted system, I am afraid you, and all others, are out of luck. No scanner will solve that monitoring issue.
Hope this helps...