There are some really great agencies out there,
As always (with bands also),lots of people playing at it,
you really need to be serious before approaching a good agent, most bookers tend not to be "facebook generation" so apart from being available, they will be looking for a website with some vid footage and pictures, they can send to potential venues,
its also very good form, to go see them in person, and agree to perform at one of their "showcases" where they will ask you to play in front of their customers.
If this all goes well, should end up with a full diary.
Its the agents job to "sell bands", not be your bestie, so they will want to book you (if you make it as one of their acts) wherever they can, so expect a few tryout gigs, before you get the premium bookings.
inevitably this will probably mean at some point if your in the UK ending up at the Holiday camp circuit,,(another topic completely), which is not for everyone, but if starting out, is good money and experiance,
you do need to be aware of the fact you will be traded like cards with other agents etc,, and may be asked to work exclusively through one office acting as your sole agent,, who would them want commission on all your existing gigs,
this can be difficult and lead to conflict, however, if your now working 4 nights a week, can be worth it.
If your just looking to pick up the odd gig here and there, its possibly not the route for you to go, if however, you have a band and want to work every weekend,, go for it.
it can lead to some great opportunities, we ended up as touring support band for a rather famous guy, and never stop touring the big UK theatres...as well as doing our own shows,,
if can help with anything, feel free to give us a shout,, and good luck with it...