I am going to assume that since you want a counter that you have seen one before that you liked correct? If that is the case and what has inspired your desire then your solution is really simple. Go to any place where you have seen a counter you like and the right click on that page. Click view source code and find the counter within the source code. Most likely you are going to see a URL somewhere within the source code that points back to the site in which they got the visitor counter from.
Being the case that I am wrong and you decided you wanted a visitor counter all on your own, or simply want to create your own, then you can as well do the same exact thing on any site that has one. Just view source codes to get an idea of what it is you need to do. I am a super novice to it all and that is what generally works for me. lol
Sorry about the slight misinformation then. It would work for the first mentioned then I guess but not the second. I have not made it that far in my studies yet and got a bit ahead of myself I suppose. Thank you for the correction. :)