barking at people

I have a 4 year old ShiTzu that barks at people when they come up the driveway or knock at the door. when they enter the house he jumps at them and sometimes he looks like he will bite them when they lean down to pet him. I read about clicker training or even using the pennies in a can. He is  very protective of his house dog. He just started running after people when he would be outside doing his business they are walking on the road or riding there bike.. His tail is wagging and it looks like he is just curious but it's getting worse. Please help!