Dont understand how to buy Bitcoin. Looked at sites but makes no sense to me I can see prices and do not understand how you buy Bitcoin. I was told you could buy as little as $1 worth.  What would you get for $100?  Nothing really helps me understand this.
Think of it like dollars and cents, pounds and pence. 

Current exchange rate is US$350/BTC = $0.35/mBTC

So $1 buys ~3 mBTC (ie ฿0.0003) . Likewise $100 buys ฿0.2895.

Use this formula:

Bitcoins bought = (purchase amount) / (exchange rate)

This is no different than any exchange rate. Check out XBT @ xe.com