Getting to USM may be tricky as Bangkok Airways built and owns the airport, you should look and see who they share award availability with to find the right carrier(s) for your trip.
Overland travel is incredibly simple in Thailand, the train system is good if it connects your destinations and there are more buses and minivans (available for charter) than you can shake a stick at. If you can get yourself to BKK, URT, HKT, or even somewhere like PEN or KUL you can get to Ko Samui in 24 hours or so, much faster if you can find a cheap Firefly/AirAsia flight to USM. Flights to USM aren't quite as cheap as other inter-Asia flights, likely due to hefty fees at USM, but I just booked USM->PEN for ~$65 one way on Firefly. (notice that's a one-way fare departing USM, more on that later)
There are many well trodden and simple routes to Ko Samui from BKK with your choice of ferry, train-ferry, or bus-ferry methods of travel.
One last word of advice: Having spent the last month and change exploring islands up and down the Malaysian peninsula and in the Philippines, Ko Samui may not be 'the island you're looking for'. Development is out of control here. There are some nice locals around, but many of them obviously resent the crushing influx of package tourists and resort-goers. This is not an island of deserted white-sand beaches and turquoise waters... perhaps it once was, but now its officially discovered and developed to death. The water is much clearer up around Ko Tao (visibility around Ko Samui is poor) and even Ko Pha-ngan is not quite as full of self-important tourists if you avoid the southern portion of the island. I think this is going to become (or may already be) the next Bali... where the Balinese are an endangered species.
I wish someone had told me to look into some of the less-publicized islands like the Ko Muk/Kradan/Ngai group, Ko Libong, Ko Bulon-Lae, or if you like a bit of development but not Phuket/Ko Samui levels of it: Ko Chang (Trat), Ko Maak, and Ko Kut.
Let me know if you need more details on getting around Thailand and in particular to and from Ko Samui and I'll be glad to share what I've learned.