Sugguest you go by train or air. Find some online train ticket agent, but I never tried. They usually charge service fee, not cheaper. Train ticket at that time is hard to buy, for the Chinese New Year. No matter air ticket or train ticket, buy as early as you can. It is 2300km from Beijing to Guangzou, nearly 21 hours by the fatest train, so it is not a good idea to travel by bus.