Car Troubles

I apparently do not know much about cars. So about 4 months ago I'm like, wow there's some weird shaking when I drive fast. I'll deal with it later. About a month ago I'm like, wow there's some squeakiness when I brake. I'll deal with it later. I am finally due for a car inspection so I go in to the mechanic. He's like, um, your one tire is down to the steel (whatever that means...I thought wheels were rubber) and your brakes are totally gone in the front (I was like but it still stops) and that'll be $390. I could have died as my tired exploded and my car wouldn't stop...who knew. Good thing there are mechanics.