To install a new template you should first choose one you like for your store's appearance. HostAndWin offers free OpenCart templates which you can look at: http://hostandwin.com/free-opencart-templates.html
Once you choose a template click "Download" and enter your name and email. The template file will be sent to the email you enter so make sure you have access and it's valid email address.
Once entered your name and email click "Receive your template" and our system will mail it to you.
Now check your mail. The email message should look like this. Click the "Your new template" link.
Save the file to your local computer.
You will find a folder with the template name in the archive.
Extract the folder to your local computer and use an FTP Client to upload the file to your OpenCart.
The template folder should be uploaded in the "catalog/view/theme" folder of your OpenCart installation.
For example if you have OpenCart installed directly on your domain "example.com", the path to upload it will be "public_html/catalog/view/theme/"
If you have OpenCart installed on a subdomain "shop.example.com" or in a subfolder "example.com/shop" the path will become "puclic_html/shop/catalog/view/theme".
Once you uploaded the template folder, log in your OpenCart administration panel.
Go to System » Settings from the navigation menu.
Click the "Store" tab.
Choose the template you uploaded from the "Template" drop-down box.
Click the "Save" button in the upper right corner of the page.
That's it. You have successfully activated your new OpenCart template.