Technically circular knitting is exactly the same. Knitting is knitting. There are various methods to use them though. -
One circular needle can be used on hats if there are enough stitches to fit comfortably around the cable and needles. (if you do this you need to have another method though when you come to the decreases)
Two circular needles can be used with stitches on each cable and you knit one side then other.
One long circular needle can be used by using the magic loop method. If you use a short cable like a 24 you need to use the single loop as Sue posted.
There are tutorials in the video section. None of them are hard they're just something new to learn. I prefer to use magic loop on hats and I prefer a long cable.. at least 32 and preferably longer. I have interchangeable needles so I have what I need most of the time.
Look up double pointed needles on Google and you'll find lots of them. I suggest you don't buy super cheap ones though. They aren't always good. I like the Clover ones or ones like the ones I linked to.