The easiest way to get Bitcoin from cash (physical bills) is called a bitcoin atm. You put the money in and it sends the equivalent bitcoins to your online wallet.
To use an online exchange you have to identify yourself for legal reasons, usually with a picture of your ID. This sounds sketchier than it is. I recommend circle to do this, but there's a $100/day cap that can be circumvented by creating more than one account. They don't cross reference names I guess.
As far as getting scammed goes, vendors have reputations. Someone who's scamming won't have a 5 star reputation and 10,000 confirmed transactions.