Trains take 15-25minmin to Amsterdam Central (look for Amsterdam CS):
http://www.schiphol.nl/_van_naar_sch...=1205654572857
There's a journeyplanner on the above site that you can look at for platforms, etc, but it's a user-friendly system once you get there. Tickets are around 4EUR.
Details on lockers are here:
http://www.schiphol.nl/_inchecken_ba...=1184589100186