at the bottom of the page when your posting a thread theres a button called manage attachments, click that and a box appears, this will allow you to navigate through your PC to the file you want to attach, double click or press open which ever and click upload file
once uploaded close the box and click submit on the thread.
Provided that the file is one that is allowed, being a tune it probably will be and also it has to be under 1MB or around this size, you should have no problems