3 ways to get there.
By Bus - Take MRT to Tiong Bahru(EW17) station and there is a special bus to pickup passenage to Sentosa(once U reach there, ask around & they will show U the bus stop). Pls don't pay the bus driver , they will drive U to entrance then U purchase the ticket($3).
By FERRY - U have to take a Taxi to world trade center Singapore, then buy the ticket ($4.30).
By Cable car - Take taxi to Mount Faber then buy the ticket . There is a Normal Cabin($10.50) and Glass cabin($17)
http://www.sentosa.com.sg/island
-admission.htm