how to edit shopping cart and checkout title and error mesag

Hi All,
Iam new to opencart and all this stuff. need your help.
I have setup minimum purchase amount to 1500 in my cart. but the error message doesnot show up at time of checkout. when user is trying to checkout below 1500.

Can you help me with this 2 queries.
1. I want to add a message saying (Minimum Purchase of Rs.1500/-) in the shopping cart and checkout page under the title.

2. How to change show up the error messages (Minimum Purchase of Rs.1500/-) when user checkout less then minimum purchase amount. both in inshopping cart and checkout.

hope i have not confused you.

could you please tell me were and what to cahgne to achieve this

thanks in advance
naveen