Hi and Welcome
Your question does not clearly define your problem.
If you wish to have a fixed IP address within your own network (as you might need with a networked shared printer) This is usually set up by using your browser to open the set up page on your router. Most routers allow fixed ip to be set against mac addresses
OR If you want a fixed external ip, ie visible to the outside world, normally you need to use the settings provide by your service provider on their website site to provide you with a static ip address. Not all offer this.
Hope this helps
It would be best to post again with full details of your router and service provider and then someone may be able to give specific instructions
Roger