You have started a thread before..did you forget how? When you go into the forum look to the left above the posts and it says "New Thread"..click that to start a new one.
Are you saying you don't know how to find your posts after you've asked a question? You can subscribe to it so you'll notified in your email or if you prefer you can subscribe and then go to your user settings (above the add on the left is the link) and it'll list any replies you've gotten to subscribed posts.
OR are you asking how to link to an outside URL? That's easy, too. Just type in or copy and paste the URL in your post. It automatically links by itself.
Does that help?