To get to your site's webmail, go to the home page of your site. In the Address Bar of your browser, after the .com, type /webmail. (Make sure that is a forward slash.)
Use your whole email address as the username, and use the password you were given. After logging in, choose a Mail program (usually Horde or RoundCube) and go from there. To keep your site unclogged, please be sure to empty your Trash bin regularly. Thanks!