Galaxy J1. When I connect to a wifi is that using my phone time and/or data all the time?

I just got the thing today. Know nothing. If I connect to our home wifi - or any wifi - is that costing me anything if I do nothing else?

Can I be connected to numerous wifis at once?

Or does it only costs me when I open a browser or something? And what does it cost me then? Just Data or phone time, too?

If I don't/can't connect to wifi will the phone connect to the internet via phone connection and cost me phone connect time + data ?