If you want to remove the "Powered by OpenCart" text in your footer, it's actually quite easy. You would need to FTP into your site, and browse the:
directory. Once you are here, you will see a file called footer.tpl. All you need to do is to open this file, and edit the lines of code that say "Powered By OpenCart".
This is the code from OpenCart 1.5:
<div id="powered">Powered By <a href="http://www.opencart.com">OpenCart</a><br /> Your Store © 2012</div>
You can simply change this to something like:
<div id="powered">Powered By <a href="http://www.theURLtomycoolwebsite.com">MyCoolWebsite</a><br /> Your Store © 2012</div>
OpenCart asks for a donation if you do this.
Hope this helps!