You apparently don't know how to send a file as an attachment.
First off, press Post Reply, You will get a text area, and down the page, you will see "Options" with different boxes some of which are ticked and unticked, And right near "Options" text, you will see "Upload Attachment", click on it, and then click on the button to choose your .rar file which you want to share, and press OK, Then press "Add file". In "File comment" area, put the password of the archive if there is one. Now, write a few explanations in the text area, and finally, press Submit button.
Hope you will learn how to do it by now.