Want to travel to Maastricht from Amsterdam and return. What type of ticket is best and where can I purchase them.
Many Thanks

You can buy your ticket at at Maastricht station. A single (enkele reis) if you are saying in Amsterdam, a return (retour) if you are planning on returning to Maastricht on the same day.
You will need to buy your ticket at the ticket window rather than at a machine unless you have the right change for the machine or a Maestro debit/ATM card with PIN.
You can't buy a ticket online unless you have a Dutch bank account with internet banking.

Have a look at http://www.ns.nl/en/travellers/home for times and prices.

topping for later comment after decades of incessant rail travel in The Hetherlands and Holland as well.

Sorry - I miss read your OP. You buy your ticket at Amsterdam station, again a single unless you are returning the same day.
If you have chip and pin credit card you can buy your ticket at the machine, otherwise my earlier comments apply. Only Schiphol and Amsterdam CS have machine which accept credit cards.