A Notified Body is authorized to test products covered by certain Directives. Whether the product has to be tested is shown in the conformity assessment.
You may also need to set up a quality system. Whether or not and under what standard you will find in the Directives.
There is also a possibility that you must only file the technical file with a Notified Body. They do not look at the file but keep it in case the authorities ask for it. For most products, you are not required to enable a Notified Body and, if so, you can often test your product through a test house.
In the CE Tool, you can easily check if this is mandatory or not and which Notified Bodies are authorized to do so.