You pay a one-time fee to each of the three credit bureaus. The fee varies by state (around $5-$10, may be less for seniors, usually free for identity theft victims). You can do this online.
If you need to unfreeze (eg. applying for a new credit card or a loan), you need to pay another fee per credit bureau, so you should find out which bureau will be used. You can unfreeze permanently, unfreeze for a short time period, or get an authorization code that you can give to whoever needs to check your credit report.
No, this is in the case of fraud, and then unlock it after 3 months. If you want it frozen until you rescind it, it costs $10 per bureau, but there may be an exception for fraud.
In addition to reading that, I signed up for one of those credit monitoring/protection sites. The one I chose was TransUnion, but I'm still learning about this stuff. I suggest you look around.
