Build! And build some more (especially 'recruits' for attacking).
Tap on the little tab at the bottom of your screen where it says your city's name (on Captivate so it might be different for other phones?)
Tap on Map and from here you can see other people's cities and the monsters around you. BE CAREFUL WHAT YOU ATTACK.
Just tap on a monster and you have a choice to Spy, Raid, Conquer, Mark or Close. It shows their number too. I've found that anything with Few written for how many there are seem to be the only ones I can conquer right now.
Hope that helps!