Click and drag your avatar to walk across the floor. If you drag slowly, you'll stop when you bump into objects in the room. If you drag quickly, you'll pass right through these objects.
Double-click any 'walkable' surface to jump to that spot (you might be surprised by which places are walkable). You can also select any object and click Move To to be transported next to the object.
To sit down, double-click a chair, stool or sofa.
You can use the arrow keys on your keyboard to move the camera around the room, including away from your avatar. The scroll wheel on your mouse will help you to zoom in and out.
Find out more about adjusting the camera angles here: