but if i could buy a joit-ticket that would be awesome!!! please tell me more
When you get there Adam you will see just how easy it all is.
As already said, ANY travel agent on Phuket (or indeed Phuket Town Bus Station) will sell you a through-ticket to the island of your choice. No need to worry about connections etc., just sit on the bus and enjoy the ride, you might have to change buses in Surat Thani, but it's all very well organised and you'll be shown exactly what to do. Really no need to worry about a thing. You will see!
FFS's link shows that the ticket through to Samui should cost you 350 baht. I don't know how recent that info is - I wouldn't be too surprised if it has gone up slightly. A joint ticket from Surat Thani airport to Ko Samui is now 350 baht, and Phuket is quite a lot further from Don Sak pier than Surat airport.