To remove the "Ecommerce Software By Shopify" (just a note - we are required by Shopify to have this in the theme, that's why it's there):
1. Go to Admin > Themes > Template Editor. Under "Snippets", open footer.liquid.
2. Do a CTRL+F and search for "Ecommerce". Around line 91, you'll see this:
<small>Ecommerce Software by <a href="http://www.shopify.com?ref=shopifyninjas">Shopify</a>.</small>
3. You can either delete it completely, or place it inside <!-- comment tags like this -->