I have a Dutch T mobile phone.
Will I need a US SIM card?
Will my phone work in the US?
What kind of charges am I to expect?
You will need to check with your Dutch-based T-mobile provider to see what your plan allows internationally to know what the rates for U.S. usage are.
I may be stating the obvious, but you could also keep your phone in Airplane mode & just use wifi to surf & make calls on Whatsapp, Facetime etc back to the Netherlands.
Obviously, check with Dutch T Mobile. US TMobile works in Europe with no extra charges for normal use-Maybe yours will work the same way.