Several things. Do you have the temperature complication showing on your watch face? I do, and if I tap on that the weather glance is launched to my default location which is my current location determined by the gps of the phone.
If you do not have the temp showing do you have Weather set as a glance on the watch app on the phone? Also in the watch app under Weather (where you can set it as a glance) there is the setting for default city. Mine is set to Current Location. Now in the Weather app on the phone the first location should be the current location indicated by that arrow/pointer. In addition to that one I have 3 other cities listed but the current location is the first and default one. The watch just mirrors whatever you have set on your phone. You must have it correct there first.
If you have the current location set (indicated by that arrow) then just tapping on that and swiping upward gives you the forecast.