In the top you see a (dark blue) bar with "Home", "Help", "Search", "Donate" and "Profile".
You click on "Profile" and than go to the left where it says "Summary". Click it and copy the URL.
Mine is: bitcointalk.org/index.php?action=profile;u=1135468
Or you can click your name after you post something..
then see the url page and the number is your uid, with u=.... < (your uid).