in designing power supply the unit should meet thr requirements fo Agency approval ,is there is any one know how to be certificated and also how to learn these stuff

 

Contact the agency. They have a long list of approvals and it depends if you are using an assembly of other agency approved parts or if you are asking for your part to be approved as a new design.

Many different agencies (UL, TUV, CSA, ...) so I don't know exactly what you are looking for. Best to contact the agency for category, price and process.

 

As a company/designer YOU decide where it will be sold, then you look up the applicable standards for that location and purchase those standards and design to those standards.

 

Or sell/licence the product to a wholesaler/distributor and let them worry about what countries they will sell too and how/if they will bother with compliance.

If your wholesaler is in one of those countries with less regulations they may not need to worry about compliance at all. Like most of the stuff on ebay. Compliance certification is becoming a little old fashioned, a bit like patents.