Using TDAmeritrade, it's a margin account. How do you be careful of not owing too much money and going bankrupt?

I worry about pressing a wrong button and poof, I owe them 1 mil. Because it's a margin account. Even owing 10k would make my heart sink. But I like what TD is offering and I like the platform.
So, how do you deal with this anxiety? What can be done to prevent this?

I won't be buying puts/options/futures yet. Just putting a small amount in ETFs and an even smaller amount in leveraged ETFs. Do they have "permissions" like IB?
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How will they let me know if I owe them money?