The first thing you need to do, is establish your eye level. Drawing the interior of a room, should be pretty straight forward. Once you have the eye level, All your lines should converge to a Vanishing point on that level. So if you were standing in a room. you could work out how high the door or ceiling was from your height. Like this.
Click to view attachmentSo for a one point perspective, looking from where we are looking at that scene, you could work out that all the converging lines will disappear to one vanishing point. Like this
Click to view attachmentNow, if you step around in the room and look at a corner, you can see that you will have two vanishing points, but they will still be at the same level. The eye level. Like this.
Click to view attachmentSo your lines will converge either on one side or the other.
When drawing things in perspective, use a ruler to keep the lines straight. Draw very lightly, then you can draw freehand over those lines to make it look more like a drawing.
Hope this helps. Any other help I can offer, I'd be only too happy to oblige.