Sorry, I must have glazed over this post at some point. Assuming you're using the built in Joomla registration, the below link should work for you:
index.php?option=com_user&view=user&layout=form
This will automatically redirect a new user to the Joomla user profile page where they can see their profile information. It won't force them to change it, but it helps. If you'd like to use a different page, because of a different profile component or whatever, the best way to find the right link is:
1) Go to the Joomla admin area
2) Click on Menu->Main Menu (or any menu)
3) Click "New" as though you're creating a new menu item
4) In the tree list, select where you'd like the user to go (for the above, I chose User->User->Default Form Layout)
5) Click Save
6) In the new menu item, where you can set the title, alias, etc. The "Link" value is what you'd use as the link
7) Hit cancel so as not to actually save the new menu item.
New User Redirection
