How to sell/buy

Hello, FA Sure here…new member.
I am undertaking my school of pipsychology here. Quick question my demo account has this currencies NZD/USD (sell 0.70255 Buy 0.70275) in that order. I want to buy the NZD, how do I go about it? given that my demo account is in US dollars.
Help me understand this sell/buy thing and base currency.