There are a couple of ways to do it.

The first uses the post buttons to link, the second uses codes.
1. Open the new post/reply window, or signature box, then type what words you want the link to say like "My Pictures". Then, go to the site you are trying to link and and copy the URL address then come to the mothering post or reply window, highlight the words "My Pictures" (or whatever you wrote) click the earthglobe with the little chainlink icon on it, and in the box that pops up paste the link in. Then click Submit New Post/Reply.
2. I'll try to type this so you can cut and paste the code to practice. Let's use google as an example.

To put the link to Google in a post you'd type
(url=
http://www.google.com/)Google(/url)
BUT, you would need to change each
( to a
[ and each
) to a
] (putting the
[ ] in the above would have shown you the link not the code you need)
Doing that changes (url=
http://www.google.com/)Google(/url) to
Google
Just substitute the link with the site you want and the words to
Look Here or whatever you choose.
I hope that helps!
Links/Linking