Prometheus is a metric scrapper, it just recollects metrics from either its own computer or another one. If you want to monitor something, you also need that something to publish metrics, so they can be scrapped by Prometheus.
Thus if you want to monitor even just a single computer, you need node-exporter to publish the metrics, and Prometheus to gather them. Then you can use Grafana to create beautiful dashboards (or use community's), and even add alarms to it.