First, associate your wireless interface with your access point. Example
Code:
iwconfig eth0 essid "<your AP>"
Substitute the actual name of your access point for <your AP>. You may then need to start DHCPCD
If it's already running, then delete the existing PID file
Code:
rm /var/run/dhcpcd-eth0.pid
Assuming that everything else is set up correctly, the above generally does the trick for me. Good luck with it. As an aside, this is a very common question and there are a lot of existing posts on it; check out some of them for other suggestions.