you've got a couple options here
what I do almost always is pour a ground plane.
1) draw a polygon to cover your entire PCB
2) using the NAME tool, click on one of the polygon lines, and name it GND
3) hit the rats-nest button and your polygon should fill in, if it hasn't already
if you haven't specified GND in your schematic, then your polygon probably wont fill. In this case, you should put a ground symbol on your schematic and connect it to something.
or, click change and set orphans to ON. now the poly will try to fill everywhere it can regardless of its signal name.
another thing you'll want to do, click change and set isolate to 0.024 (or more/less depending on your confidence level). this setting tells the polygon to stay at least N mils away from other signals, pads, holes, etc.