Message setting

Dear All SAP Gurus,

Can anybody tell me about message setting? How to set the message setting in MM or in general?

for eg. If i entered the invoice date prior to po date in J1IEX, system is giving warning message , i want to change it error message .

my quaestion is that how to find a particular message means 8I 194.How to to attack on specific message.It may concern with purchasing,inventory, material master or cin like that.....

Or if i want to set new message please guide in this matter.

Thanks and regards

Mahesh