Of course the Mandarin Chinese is more popular and used. As far as I can see, Cantonese is only used in Guangdong and Guangxi province and so on.

Mandarin is the official standard. It is based on the dialect of Beijing. So although they are supposed to, most Chinese people outside Beijing don't speak Mandarin in daily lives. But that doesn't mean they don't understand Mandarin. Learning Mandarin is required in most primary schools. Even if you didn't learn Mandarin formally at school, you always have all the TV and radio broadcasts around you, which are mostly in Mandarin. Considering the pronunciational differences, there are numerous dialects in China. However, most dialects in northern China are quite similar to Mandarin, so the transition is not difficult. But dialects like Cantonese, which is only used in areas like Guangdong and Hongkong, or the dialects spoken in Fujian Province and Taiwan are fairly different from Mandarin. That means those dialects are quite difficult, if not impossible, for people from other places to understand. If you want everybody understand what you're saying, you'd better speak Mandarin. Althoug it doesen't matter If you only live in Cantonese spoken areas.

almost every can speak mandarin but only people from guangdong speak cantonese

If you are interested in dialects, don't forget there is one called '闽(min\/)南(nan/)话(hua\)'. I love it! Although I can't speak it.

Cantonese is only one of more than 80 sorts of dialects in china. More common talked, there are seven categorys. The seven dialects cover more 70% people. Cantonese is just one of the seven major dialects. For these dialects, most northern pronunciation is near and could be understood each other even for different dialects users. Mandarin is popularized as offical pronunciation of China so most people could understand. Another point is that people will feel you are more close and intimate if you just talk with him through his dialect. They will be evan exciting and more friendly to you. However, if the dialect tone is unacceptable for them, that will be very embarrassed. So Cantonese is a good tool for your communication with people in Guangdong or Hongkong areas. If your friends spread in China, you had better to study Mandarin.