Welcome to the Cesium community!
If you have a longitude, latitude, and height, you can fly the camera directly there like this:
viewer.camera.flyTo({
destination: Cesium.Cartesian3.fromDegrees(20, 10, 1000)//lon, lat, height
});
From the doc: https://cesium.com/docs/cesiumjs-ref-doc/Camera.html?classFilter=camera#flyTo.
To get the user’s location in your app, you can use the browser’s geolocation API, see https://www.w3schools.com/html/html5_geolocation.asp.
What kind of project are you working on?