Re: samsung s3 just getting flashing lines on screen
If it's a brand-new device it should be covered under warranty - the store should be able to help you with a replacement phone, unless you dropped it, got it wet, or rooted it, in which case your warranty is void.
If you bought it used you have no warranty.
Without knowing any information other then what you've given, I would say that your digitizer (display) has failed and it will need to be replaced. Prices will vary depending where in the world you live. Check with local electronic repair shops to see how much it would cost to have the problem diagnosed and repaired.