If you're in a hurry, cab it -- but it will cost you, possibly up to $50 depending on traffic. So if you don't mind taking the scenic route, I'd recommend taking the CTA's Blue line train from the airport to downtown's Washington station. From there you can either hop in a cab or catch the 20 or X20 bus to the stadium. Fare shouldn't be more than $5 one-way, though it will take you an hour and change.