Sony Ericsson Xperia Play — Whats the defalt Wifi or data plan

This might have been asked already but whats the defalt for an xperia? I have wifi at home and I want it to download from there when I have a connection. Do I have to tell it to connect each time I want it to use wifi? How do I know it wont use my data plan because that costs money`?

If you have both WIFI and Data on the phone will automatically switch to WiFi when there is an available and saved single like the one at home. So I wouldn't worry about it if I were you.