Behold the power of the Admin
But, seriously, there is nothing strange or wrong with somebody knowing your IP address. For example, z3r010 knows your IP address as well. So does every other administrator of any other site where you're registered, if they ever bother to look for it.
This is a good example that dispels the myth of web anonymity: the exchange of IP addresses is built-in into the TCP/IP protocol. To describe it in lay terms, if websites would not know your IP address, they would not know where to send their info and you would not be able to surf the web.
Now, what can be done to you by a malicious hacker depends ultimately on the balance between the hacker's skills and your defenses. The best you can do is to get a modern router with built-in hardware firewall and manage it appropriately (i.e. read the manual). It would also help to disable certain Windows services, such as Remote Desktop, Remote Registry, NetBIOS, and so on. This is because any remote connection needs some sort of receiving software running on your computer and these are common Windows features that may be used for that purpose. You can also use a software firewall, such as a good internet security suite, to monitor outgoing traffic, i.e. have it warn you with a pop-up window every time a new program tries to access the internet. This is very helpful in case the hacker wants to plant a trojan on your system.
Finally, and this might be a wrong hunch, but I have seen ads on various, let's say, less reputable websites that state exactly what you said in your post - that someone knows your IP address (they might even show your actual IP address in the ad, to look convincing) and may do bad things to you, unless you click the ad and install some sort of protection or scan for threats or something like that. Such ads are mostly fake - they do not bring you any useful software, but rather may themselves plant malware on your computer. If you ever see such a pop-up or a flashing ad somewhere - ignore it.
P.S. You can also change your IP address. How you do it depends on your ISP, if you are using your home computer and on your network admin if you are connecting from your workplace. In the latter case, just talk to your admin. In the former case you may want to call your ISP and find out how the IP address is assigned to you. What Ciara has described is very common for residential customers, if I simply turn my router off and on, I will get a new IP address, can do this many times a day. If your connection is different, as suggested by z3r010, then call your ISP and if you need further help, let us know.