Hi, whats the cheapest mode of transportation will i take to reach guiyang city ? Train,ferry or bus

Well, it depends on which city you start from.
But generally speaking, taking a train or a long-distance bus is usually the most economical way. :-)

By the way i have change plans from hongkong to guangzhou international airport what train should i take and can i reserve tickets on line do you know how much would it cost

You can take train to get to Guangzhou East Railway Station first. The duration is about 2 hours. The ticket fare of first class seat is about 230 HKD per person.
After arriving, you can take subway line 3, get off at Airport South, Exit C directly.
The ticket fare is CNY 7 per person.

As I know, the official website doesn't have English booking page. You can ask help for TravelChinaGuide. They provide online booking tickets service. You can trust them. The contact information is bookingtravelchinaguide.com.