You can't buy shares for the new GM that has emerged from bankruptcy. Because it is now privately owned by the government and the autoworkers union. But rumor has it that some time next year or in 2011 they will issue shares and start trading on the stock market.
GM has been split into two different companies. The remaining old GM has been left with a lot of debts that were accumulated over the years. And for this reason shares for this old GM are now worthless. But some ignorant investors might still want to pay good money for them, when the trading resumes for these shares of old GM.