take a look at the following examples
http://threejs.org/examples/#misc_controls_orbit
http://threejs.org/examples/#misc_controls_trackball
there are other examples for different mouse controls, but both of these allow the camera to rotate around a point and zoom in and out with the mouse wheel, the main difference is OrbitControls enforces the camera up direction, and TrackballControls allows the camera to rotate upside-down.
All you have to do is include the controls in your html document
<script src="js/OrbitControls.js"></script>
and include this line in your source
controls = new THREE.OrbitControls( camera, renderer.domElement );