I hope this will be an easy one:

When I turn the water on in the bath, the water flows out from the faucet. Then I would pull the pin on top of the faucet and it will redirect the water to the showerhead.

Now, when I pull the pin, the water just continues gushing out from the faucet, in a different pattern. Pressing hard on the pin will send some water up, but not much, and that uses up a lot of effort.

From what I can tell, the pin has a plastic attachment that is in the faucet and pulling it up will plug the hole and send water to the shower head. Is it broken? What's the deal? How can I repair this thing? Obviously, I am really clueless about this so a reply like "change the pin" isn't getting me anywhere. I will need more info on how to do it, and where to get the parts from and so on.

Thanks a lot!:)

This is likely a delta faucet, you need a replacement diverter. It simply screws in under the cover plate.