Well I use a laptop which is connected to the internet through a Wifi router and everytime i check my IP it is different I want to have a static IP.
ALmost all residential/home Internet Services manage IP's using the DHCP system that reissues IP addresses every so often. How often, what IP's, that's all controlled by the ISP, as Wimiadmin said. Most of these ISP's will issue static IP's on request, although a pretty large fraction of them charge you for it (some with an extra fee, some by requiring you to purchase "business class" service).
The reasons for this can become a fairly large discussion, but it boils down to what Wimiadmin said: talk to your ISP.