Is there any way to convert any other crypto currency into ADA?

I want make a POS(Point of Sales) system for Cardano to accept payments, in which I need to convert other coins (e.g. ether) into ADA and store those ADA funds into Cardano wallet.

I want to make something like this, here is example Changenow coingate

please suggest me any idea related to how to exchange currencies and how to do it

There are several big exchanges that have API’s available which you can use to exchange / convert coins. But if you have enough traffic, you could also just connect supply and demand yourself.

For example Kraken REST API Documentation