I want to know how to spoof IP address?

The answer to - how to spoof IP address is - you can't. Whenever you need IP traffic to be bidirectional, IP spoofing is no use. The contacted server would not reply to you but to someone else, the address you spoofed.IP spoofing is then normally "useful" only to disrupt communications - you send harmful packets, and you don't want them being traceable to yourself. In specific situations you can use a double spoofing to gather a measure of bidirectionality. For example let us suppose that we know of a system somewhere that has poor sequence generators - whenever you send a packet to it, it will reply with a packet containing a monotonically incrementing number. If nobody was connecting to the system except you, you would expect getting 1, 2, 3, 4... . Now let us further suppose that you're interested in whether another system is replying to specific packets (e.g. you're running a port scan), and you wish to receive some information but don't want the target system to have your real address. You can send to that system a spoofed packet pretending to be from the poorly-sequencing machine. Now there are three possibilities: the target system does not reply, it replies, or it actively counterattacks and (e.g.) scans the pretended source to determine the whys and wherefores of that first packet. 

What you do is, you scan - without spoofing - the poorly-sequencing machine (PSM). If nobody except you has connected to it, which means that the target machine hasn't replied to the PSM, you'll get 1-2-3-4-5. If it is replied once, you will get 1-3-5-7 (the packets 2, 4 and 6 having been sent by the PSM to the TM in response to the TM-PSM replies to the spoofed packets from you to the TM. If the TM makes more connections, you'll get something like 2-11-17-31 or such. The PSM knows your real address, of course, but the TM does not. This way you can spoof a connection and still gather some information. If the PSM's security level is low enough, this, combined with the fact that your "scan" of the PSM is harmless, is (hopefully) enough to prevent consequences to you.

Another possibility is to spoof a nearby machine. For example you are in network 192.168.168.0/24, have IP 192.168.168.192, and you have promiscuous access to some other machine address space, say 192.168.168.168. You just have to "convince" the router serving both you and the .168 machine that you are indeed the .168 machine, and take the latter offline or disrupt its communications (or wait until it is offline for reasons of its own, e.g. a colleague logging off for lunch). Then the replies to the spoofed .168 packets will sort of whizz by past you, but as long as you can sniff them while they pass, and the real .168 isn't able to send a "That wasn't me!" reply, from the outside the communication will appear to be valid and point back to the .168 machine. This is sort of like pretending to be your front door neighbour, while that apartment is really untenanted. You order something through mail, the packet gets delivered to the other's front door, you tell the delivery man "Oh yes, mr. Smith will come back in half an hour, I'll just sign for him" and get the package.