You need to obtain the weather information, using any means available to you, on the phone side and then send that data to the watch (if you want to target a specific watch) or, using DataApis, sync that data across the whole network of nodes. Then on the watch side, you need to have a component (can be part of your watch face) that can listen to changes in the data and capture the data that was received and show that in the watch face. In other words, you cannot get the weather data directly on the watch, you need to use the phone component as a proxy or so. Each of these steps can be found in the official docs (for example for data exchange see here) or among our official samples on GitHub.