As root (or with sudo)
# ifconfig eth0 192.168.1.2 netmask 255.255.255.0
Where eth0 is the interface name "ifconfig -a". And 192.168.1.2 is the IP you want to change it to. Although many times DHCP sets that up for you. With dhcpcd or dhclient depending on what your distro has and what you prefer. There's also routing that need to be setup. Which is handled for you with DHCP. There's also "ip" which is the new way of doing what ifconfig and stuff does, but I'm not that versed about it, other than it exists. And I sometimes need to use it to force IPv4 on the wifi device.
# route add default gw 192.168.1.1
Although it's more likely that dhcp is setup.
# dhclient -v eth0