I've toyed with them a bit (pardon the pun), but nothing significant (less than $100).
First, you find an exchange provider that converts your cash to bitcoins and vice versa. Lesson one: different providers take different fees.
Most exchanges work online, but there are some physical ones too. For example in Finland there are 5 locations where teller machines (similar to ones used by banks) are available. If you want to turn cash into bitcoins, you go to machine, enter bills in, and in the end choose to transfer equal amount into your bitcoin wallet (authentication is done through mobile phone/app). And if you want to withdraw bitcoins to cash, then you enter your bitcoin wallet details and get real world money out of the machine. Lesson two: the more physical the service, the more you pay.
You'll also need a bitcoin wallet. This is essentially a tool for managing your bitcoin account/assets. Again, different providers, different levels of security, different costs. Lesson three: loose your wallet, and the money is gone. Forever (no ifs, no buts).
Likely there's a ton of stuff I'm not aware, so take the above "as is". This helped me to get the basics:
https://en.bitcoin.it/wiki/Help:FAQ -k-