To follow a post or announcement you must be signed in. This is because the alerts will be sent to you via email with the email you provide when registering.
When you have signed in and located the post or announcement you want to follow you should see an option to Follow on the right-hand side.
Once you have clicked this a banner will appear at the top of the page to inform you that you are now following the post.
If at any point you wish to unfollow the post you can return to the post and click unfollow.
The same banner will appear at the top to confirm you are no longer following.