I would say: a) I'm just kidding. or b) I'm only joking. You don't need the "you." Another expression that means the same thing is: "I'm pulling your leg."

"Joking": "I'm joking you" is always incorrect. Correct: "I'm joking with you." "I'm joking." "I'm just joking." "I was only joking." "We were just joking around." See my reply post re: "kidding" for more explanation on the differences between these.

"Kidding": "I'm kidding." "I'm just kidding." "I'm only kidding." "I'm just kidding around." "I'm kidding you." "I'm kidding with you." There is a slight difference in meaning between "kidding" and "kidding with" and "kidding around." "I'm kidding" implies that someone said something untrue or unkind and, afterward, wants to claim that they said this merely in an attempt to be funny. "I'm kidding with you" may mean the same thing; but it may also mean something more like "I am trying to make you laugh or be less serious," with or without using words. For instance, imagine someone using their toe under the table to mimic a bug on someone else's leg. The second person squirms each time and finally jumps up, shouting, "I think there's a spider on me!" The first person slowly smiles and says, "I'm just kidding with you." Notice that this involved a prank where words were not spoken. So "kidding with" may or may not involve words. "Kidding around" usually focuses on actions more than words, and the actions are generally perceived by others to be hurtful, irresponsible or otherwise negative. Imagine a mother hears a vase crash to the floor in the living room. She rushes in and one of her two young sons says, "We were just kidding around." When someone makes the statement or asks a question, the meaning can change: "Are you kidding me?" "You're kidding." "You've got to be kidding me." Usually, these are used idiomatically as an exclamation meaning that something unexpected (good or bad) has just happened. And there is not always an object. For instance, someone may scratch a lottery ticket, see that they've just won $1,000 and say, "Are you kidding me?" Or imagine that someone is driving home on a hot day, her car air conditioner stops working, she notices that she is almost out of gas—and then she sees sirens behind her and realizes that she is being pulled over by the police. In her frustration at her situation, she may say aloud, "You've got to be kidding me!"

you can say I'm kidding with you or I'm joking with you

1. I'm kidding with you. 2. I'm joking with you.