When you arrive in Samoa at Faleolo International Airport, there are now a few money change facilities there that you can use. Western Union has pretty good rates. ANZ and Westpac have booths in the same area. There used to be just one ATM on 'Upolu, but over the past few years these have now popped up in other areas. On the way from the airport into Apia you will see a few ATMs on the side of the road. There are now quite a few in Apia township at the main banks.
Some places on 'Upolu have VISA and other credit card facilities, but many also do not, and deal only in cash. Make sure you have enough cash on you if you're going tripping around the island.
On Savai'i, there is definitely only two ATM, and both are in the small town of Salelologa, where the ferry lands. Not very many places on Savai'i have credit card facilities, so make sure you have enough cash or you'll be begging a lift back into Salelologa to get some! EFTPOS is rare.