Bangkok Airways offer 18 daily direct flights to Koh Samui, with the first flight departing Bangkok at 0600 the last departing at 2200. The best option is to extend you international Ticket to Samui Airport.
Regarding flights to Samui, Bangkok Air has momentarily the sole and exclusive rights to fly to Koh Samui although Thai Airways now operates two flights per day as well between Bangkok and Koh Samui.
You can book flights directly on Bangkok Airways website.
Note that the first and the last flight of the day have special rates, almost 50% cheaper than the standard rate. If you sign up for their Flyer Bonus program, you receive an additional 10kg luggage allowance, amongst other benefits. Signing up for the Flyer Bonus program can be done here.
Overnight sleeper from Bangkok Hualampong Station departing between 1800-1900 arriving in Surat Thani 0600 where there is a bus shuttle and ferry link service to Samui
Overnight VIP Sleeper Air con Bus departing from the Southern Bus Terminal around 1900 arriving in Surat Thani at 0700 where there is a bus shuttle and ferry link service to Samui or by private charter companies/travel agencies at Kao San Road in Bangkok with a direct connection onto Samui by ferry.