If you're
(a) logged in ; and
(b) viewing your own thread (rather than one created by someone else)
there will be an option to 'Mark as Best Answer' next to each contribution on that thread. Simply click on the one of your choice. (If someone posts a better answer later on you can change your selection simply by clicking on 'Mark as Best Answer' against the new post).
Most people who post here go back to look at the threads they've posted on, so simply adding an 'answer' on your own thread enables you to post a message of thanks to them.