how & where to travel in London or the near-by area,eg Paris if i only have 500 pound?

I would not cosider Paris to be in the 'near by area' of London, but if you have 500 pounds then contact Rail Europe. www.raileurope.com and by a rail pass.

I woulld check out raileurope as your best bet if you are considering travelling in Europe. You can get good information on London and travel in to Europe at http://www.londonvacationguide.com