There are loads of ways to cast on in knitting.  Some people just prefer one particular method, others use different methods depending on what they're knitting - some methods give a more elastic edge than others.  But the main thing with knitting it that whatever works for you is fine - there is nothing right or wrong about any particular method.

The Knit Cast-On is the easiest for a beginner pick up as it uses the basic knit stitch so there is less to learn.

Begin by making a slip knot and pop this on one knitting needle- this is your first stitch.  Pull it snug to the needle but not too tight - far better have a loose cast on the a tight one. 

Holding the needle in your left hand, put the other need through the middle of the stitch from front to back. 

Now, wrap the yarn round the right needle (under, then over). 

Pull the right needle back making sure that the wrapped yarn stays with you (you are basically using the needle to pull a loop of yarn through the stitch). 

Finally, hook this new stitch back onto the left needle without twisting it.