There is a ferry service which connects the mainland of Thailand with Koh Samui. How to get from Koh Samui to the mainland and back - read in this section, there is plenty of options. To book accommodations in Koh Samui there are links below:
Choosing and booking a hotel room in Koh Samui - "Koh Samui Hotels"
Choosing and booking a house in Koh Samui - "Rent a house in Samui"
Choosing and booking a villa in Koh Samui - "Villas for rent in Koh Samui"
Choosing and booking an apartment (condo) in Koh Samui - "Apartments for rent in Koh Samui"
We also recommend that you read the Samui beaches section.