1. Joking with me, or trying to fool me into believing something. 2. Something that is easily available. They are not rare.... 3. It became clear to me or I realised.

These are all common idioms. 'Pulling (someone's) leg' means teasing someone. If you say 'I'm pulling your leg' it means that you're joking/kidding/not being serious. What you have just said to this person is not true - you just said it as a joke. 'A dime a dozen' is an American expression meaning that something is very common, or that there is no shortage of them. If you can buy a dozen of these things for only a dime, they are obviously very cheap and not in great demand. For example, if you say that you are going to a big city because you think you'll find a job as a freelance DJ there, your friend might tell you that freelance DJs are 'a dime a dozen' in the city, meaning that there are plenty of them and you might find it hard to find someone to hire you. 'I dawned on me' means that I realised something. It's an image of the sun rising in the morning, making things clear to you.