The plank won't help you lose weight but can help you build up pain endurance. Just don't assume that it will give you a toned stomach, you need to diet, do cardio and weights for that.
Regarding the plank, do it on your arms and feet. It doesn't matter if you cannot do it for very long. A few months ago I could only do a minute but, as my fitness improved and I was able to do more sit ups, I managed to get 4 minutes 15 last time I timed myself. Do the plank several times a week and try and hold it for a few seconds more each time. Do it on the floor,as the mattress on your bed will be soft and affect your plank. If you are worried about carpet burns, as some people said, use a yoga mat.
Hope this helps, but remember plank alone will not give you a toned stomach.