Koh Samui is a small island.  There are many beaches, a few towns.

The best thing any traveller can do is to read and research as much as possible.  Check out what suits budgets and preferences.

Once we had to go to a book store or library to find out the best travel guide, its not really necessary now a days!  here are some great websites that are really worth reading:

Here are the links to the FAQ Favourite things to do . . .

Lists from others of their favourite 3 things to see and do when on Samui -> Top 5 things to do on Samui 

Lists from others of their A-Z of things they like to see and do when on Samui -> A-Z of things do do on Samui

Official TA list of things to do on Samui -> Trip Advisor list of things to do on Samui 

Markets or walking streets on Samui -> Markets on Samui

Samui Island Explorer are great for tours and ideas -> Tours and Trips

I Love Koh Samui is also a good read 

Vitual Tourist Things to do on Samui is also packed with useful information from other travellers.