In the far right of this section, look for the Browse button. Clicking on Browse will allow you to select the first attachment from your hard drive:
Once you've selected your file, click on Upload, and you'll see the following confirmation that your attachment has uploaded successfully:
To select additional attachments, repeat the above process. You'll be able to view and manage all your attachments via the Manage Current Attachments drop-down box:
Once finished, click on the Post New Topic button, and your attachments will be available for others to view/download.
Once you've created a new post, you'll probably want to receive notification when someone has posted a reply. For instructions on receiving email notification, visit the next thread: How to get instant notification when someone responds to your post.