This is one of those questions that's been answered several times over the years. Do a search for "Loop" in all actrive posts and everything over the last few years. You will find several methods for doing this.
It;s one of the most commonly asked questions here.
[Jose Burgos]"Also if it is one of the most asked, is there a tutorial on your web page?"
Most people don't realize this, but if you click on the particleIllusion cow banner at the top of this forum, it will take you to the wondertouch tutorial page. There's also a link to the faq on that page.