There are companies in most countries that will sell you Bitcoin in exchange for the local currency. In the United States, a company called Coinbase will link to your bank account or credit card and then sell you the coins for American dollars. Opening an account with Coinbase is similar to opening a traditional bank or stock brokerage account, with lots of verification of your identity needed.
For people who do not want to reveal their identities, there are services like LocalBitcoins that will connect local people who want to buy and sell Bitcoin for cash, generally without any verification of identity required.
To start accepting Bitcoin is even easier. One needs only to create a Bitcoin address, which can be done anonymously by anyone with internet access.
The price of Bitcoin fluctuates constantly and is determined by open-market bidding on Bitcoin exchanges, similar to the way that stock and gold prices are determined by bidding on exchanges.