What's the budget?
Well, then buy an interface, disassemble it, try to paint the schematics and the identify the components. Then rebuild it. If you have too little knowledge, learn.
Maybe it would be a good startingpoint to actually describw what you want to, if you have done any research on your own and what it is that you have problems with. If you want a "gimme all", then be prepared to be asked for a bunch more bucks than you want to spend.
and somehow code the arduino in such a way that when i will connect it to pc it will show as an audio usb interface .
There are specialized chips for this. Here's the 1st one I found. "USB soundcards" use these kind of specialized chips and that's why you can buy a complete USB soundcard for less than $5 USD, or you can get a Behringer USB-guitar interface for about $30. These things don't have a programmable microcontroller.You can get microcontroller chips with a "real" USB interface but writing your own firmware to work with the standard Windows or Mac USB audio drivers is "advanced programming". Writing your own firmware and a custom driver is even more advanced.If you want to do digital signal processing you can also get spealized DSP chips. But again this is advanced programming, some of these chips can be expensive, and you might have to buy a development kit for a few hundred dollars. I assume most digital guitar "stomp boxes" use DSP chips. I'm pretty-sure most inexpensive effect boxes are analog, or maybe they use some very-simple digital processing (perhaps without a microprocessor). P.S.It's not practical or economical to build something like this. They have a team of electronic, mechanical, and software engineers and automated production equipment. The fixed-costs are high but the variable (per-unit) costs are low. They buy the components in bulk, the custom parts are manufactured in bulk, and assembly is done with very-little per-unit labor, and it's low-cost labor. For the same reasons it's not worth repairing a unit like that. Diagnosis & repair typically requires an hour (or more) of expensive-skilled labor, and if the repair is not done at the factory the replacement parts have to be purchased & shipped in small quantities.