|If you are referring to your public IP address which appears you are, you don't do anything. You contact your ISP and ask them to provide you with a static IP address, you cannot just use any old IP address. That will probably cost you money.|
However, the cheaper option, i.e. free, is to use a Dynamic DNS service like DynaDNS. from them you will get a URL that is linked to your IP address. The IP address linked to the URL is automatically updated when it changes.
However, if you mean a private IP address you disable DHCP on the router and enter the IP address in the network configuration on the computer. Makes sure the address is in the same range as the private IP address of the router, usually, 192.168.1.1. you can confirm it by doing IPconfig from a command prompt and reading the default gateway address.