I am a beginner with QGIS too so it helps me to write things up.
I already upvoted dangowans' answer but here are more specifics for QGIS 3.2, too.
I am adding a service for a Bare Earth Hillshade for Oregon, see https://gis.dogami.oregon.gov/arcgis/rest/services/Public/BareEarthHS/ImageServer -- that is hosted on ArcGIS ImageServer.
There is a set of links at the top of the page "JSON | SOAP | WMS". Right click on WMS and copy the link location. In QGIS under Layer -> Add Layer choose Add WMS Layer (or type Ctl-Shift-W); this should open a Data Source Manager dialog. Click "New" and paste the link into the "URL". Give the connection a meaningful name; I used "DOGAMI" for this one.
You can generally leave all the other options at their defaults but I chose a different projection for my project to better match my other data (OGIC).
Click OK. Back in the Data Source Manager, click Connect. If everything worked you should see a list of what services are available on this server.
Select a service by clicking on it, then click Add. It won't add the layer unless you click Add. (Close or double click won't do it.)
Here is the full WMS service URL, the one you paste in should be similar: https://gis.dogami.oregon.gov/arcgis/services/Public/BareEarthHS/ImageServer/WMSServer?request=GetCapabilities&service=WMS