As an example, if you're building basic WordPress sites, you can get this done for a few hundred dollars, so that's your baseline - you don't want to be charging much more than that. If on annual salary you could earn $40K, then your adjusted hourly rate is about $26 working full time. Let's say you can only bill about 20 hours per week, then your billing rate is $52 per hour.
That means that to keep your prices reasonable, you would either have to be able to finish a WP site in about 5-10 hours, or charge less per hour. But now at least you have a pair of numbers that you can use in your calculations.
Research your competition. See what they charge and what services they offer. You can use that as a pricing start.
Contrary to most of the advice you'll receive, I do not recommend time-based billing.
Your training and skill level has a bearing on your ability to do the work. However, this has no bearing on the amount you should charge except that if you can't do the work within the desired time-frame you'll not be able to charge anything.
The price to charge should be relative to the value the client tells you they will derive from you doing whatever you do.
Negotiating some arbitrary hourly rate reduces you to a commodity, and you are not orange juice!
Make sure it's worth it to YOU. When you're doing freelance work, several things are true:
You need to factor these things into your compensation. For me, I use a simple formula: I take whatever I'm getting paid at my day job (or what I would ask for at a day job) and double it.
Once you have your rate figured out, strictly stick to it and don't accept less.
I like the value-focussed answers here.
In fact personally I intend never to return to hourly rates.
I love this apocryphal story about Picasso: a man went to Picasso in a restaurant and said, "Wow, you are my hero. I love your work. Would you draw me a little picture right now? I want to give it to my son." As he was wont to do, Picasso whipped up an incredible little sketch in under five minutes, and handed it the man with the words, "That will be €20,000" (or the equivalent currency of the time). The man was shocked and asked, "How can you ask for €20,000 for five minutes of your time?" Picasso replied, "Yes, five minutes of my time, and forty years of one-pointed dedication."
Value who you are, what you have learnt over a lifetime, and don't give it away too easily.