The time to first byte of my site is arnd 13 secs, im using opencart. plz help.
Your best bet is to search around the opencart forums a bit and ask there: http://forum.opencart.com/viewtopic.php?t=11301
First-byte performance is going to be a function of your hosting and the back-end code (including database indexes, etc). Without knowing a whole lot more about your setup I’d guess you have a combination of slow hosting (a shared server or low-end VPS) and a lot of database queries.
Fixing it will probably require tuning the database (indexes, etc) and possibly getting beefier hosting.
I had a similar problem with an e-commerce site that I was developing where I was continually experiencing a huge time-to-first-byte. After a great deal of head scratching, I found that the real bottlenecks occurred when the cart had to resize images on the fly. IE If the cart needs to display an image which is 30 pixels x 30 pixels on screen, make sure that the supplied image is that size, and not 60 pixels x 60 pixels or some other dimensions.
Another thing that helped me enormously was to ensure that the Charset variable is defined - this saves the server having to work out an appropriate Charset would be for rendering the pages. If possible, define the charset in .htaccess as this will cascade to subfolders. See How to specify Charset in .htaccess files
It is preferable to doing it this way (as opposed to defining it in the HTML) because a Request Header setting will take precedence over the same HTML setting.
Hope that this helps
Miles