With the $30 T-Mobile plan, what happens when you run out of minutes?

So I made the switch, now I'm just worried about minutes.

Most of the time I'll get away with 100 minutes, I'm not a talker... but since it's a monthly plan, what happens if I need more minutes? It does happen from time to time.

Do I just login to my account and refill early? How does that handle data?

Do I get charged per minute? Can I just login and add another $10 for minutes if I need to?

Please explain. :)