Although spoken Mandarin Chinese is standardized around the world, two different writing systems are in use. In mainland China, simplified characters are used, while in Hong Kong, Taiwan and Macau, traditional characters are used. I recommend that you start with simplified characters. Not only are simplified characters a little easier to learn, but mainland China's political and economic status is also rapidly rising, spreading simplified characters to overseas Chinese communities like Singapore and Malaysia. Once you reach a certain level of proficiency, you can learn traditional characters without too much difficulty, but in the beginning it is best to stick with one system.