Melt the white chocolate in a microwave, cooking on high for 30 second increments, stirring thoroughly with a fork between intervals. Alternatively you can melt the chocolate in a double boiler (a metal bowl over a simmer pot of water).
Matcha is a powdered green tea that you can get at Japanese or Asian grocery stores, well stocked grocery stores (look in the tea section) or online. Don’t bother getting the higher quality matcha, you can usually find culinary grade matcha at a lower price point.
When selecting white chocolate, I usually recommend looking at the ingredient list for the bar or bag and making sure that one of the first ingredients is cocoa butter. If it’s vegetable oil, its subpar white chocolate and you’ll be able to tell with the end product.