Sir my apple iphone3g's left side is not working what should i do

Try doing a software restore. If that doesn't work, your touchscreen may be faulty. Take it to a genius bar and have them diagnose it and replace it under applecare (if you have it).

If it's faulty, and it's not covered on warranty. You can buy a digitizer replacement on ebay, however I'd let a repair shop install it because the ribbon cables inside are broken very easily. If you want to do it yourself, there are many guides on youtube that explain the process.