Without a cell phone number, you can't call any of your friends; similarly, your computer needs an IP address to connect to the internet. There are many different ways you can use to check your machine's IP address. You can use the old-school (yet still popular) ifconfig command followed by the name of your network interface that is connected to the internet:
root@ubuntu-linux:~# ifconfig wlan0
wlan0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 172.16.1.73 netmask 255.255.255.0 broadcast 172.16.1.255
inet6 fe80::3101:321b:5ec3:cf9 prefixlen 64 scopeid 0x20<link>
ether 10:0b:a9:6c:89:a0 txqueuelen 1000 (Ethernet)
RX packets 265 bytes 27284 (26.6 KiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 165 bytes 28916 (28.2 KiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
You can also use the -a option to list all network interfaces:
root@ubuntu-linux:~# ifconfig -a
eth0: flags...