How to find the IP address of my computer?

Summary: Determining your own IP address is very easy, but it first requires understanding which IP address it is you want to see.

How to find the IP address of my computer? Is there any command?

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Sure there are commands.
But I have to ask you a question first:
Which IP address do you want?

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Your Computer
Each machine connected to a TCP/IP network is assigned an IP address; this we already know. It's not that difficult to determine what that IP address is.
In versions of Windows from Windows 2000 and later, run the Windows Command Prompt (Typically in Start, All Programs, Accessories). In the command prompt, type IPCONFIG and you'll get something similar to this:

Windows IP Configuration

Ethernet adapter Local Area Connection 2:

Connection-specific DNS Suffix . :
IP Address. . . . . . . . . . . . : 192.168.1.5
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.1.1

"... since I'm behind a router, this is not the IP address that will actually be used when I access the internet."

As you can see, the IP address of this computer is listed as "192.168.1.5".
(In Windows 98 and 95, you can Start->Run a program called "winipcfg" and get roughly the same information without using a command prompt.)
Now it's important to note that this is the IP address of the computer. In fact, since I'm behind a router, this is not the IP address that will actually be used when I access the internet. The IP address shown here is my computer's IP address on my local network only.

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Your Internet IP Address
The IP address assigned to your internet connection is most easily seen by visiting any of the many "show me my IP address" pages on the internet. The one my own most often is advcomp.co.cc, but there are others.

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Why are they different?
Sometimes they're not. If your computer is connected directly to the internet, then your machine is assigned an internet IP address. The IP address you see using ipconfig will be the same as that which you see reported by the "what's my IP" sites.
If you're behind a router (or a modem that is acting as a NAT router) then the internet IP address is actually assigned to the router. The router then hands out "local" IP addresses to the individual machines connected to it. Those are the "192.168." addresses you'll commonly see when running ipconfig. In fact, "192.168.x.x" is an invalid internet IP address, as that range, and a couple of others, are reserved exclusively for local network use.
When you're behind a NAT router, "ipconfig" will report your machine's local IP address, and the IP reporting web sites will show you your internet IP address.