1) Do not use any html in your post. Include the URL, just as it would appear in the address bar of your browser. For example: http://www.freerepublic.com . Free Republic's software will automatically convert it to html, and make it a clickable link. (I put a space after it, because if I don't, the period gets included as part of the link.)
2) Use an html tag for the link. For example: <a href="http://www.freerepublic.com">http://www.freerepublic.com</a>. If you use this option, you must use html tags throughout your post. For example, I had to use the paragraph tag in this post, or it would have been just one big paragraph.