I'm trying to sell my own software. I am basing this software on a software that exists already. I just want to know what needs to be different in my software from the software that it is based on. I don't want to break any copyright laws so I would like to know what factors needs to be different so I don't get in trouble.
what do you mean by ''based on a software'' ?? Because you can't edit the source of another software and sell it, except if it is a ''Open Source'' software
There are many aspect that need to be unique such as the coding/user interface/algorithm/logic sequence.
Very few clone applications are successful. If you want to sell something that already exists become an affiliate or a reseller. If not, then figure out how to do something better, don't just copy something that currently exists.
There needs to be something different, and the best way to figure out 'what' is to ask a lawyer. If you put your own flavor (and make sure certain stuff works differently) in the software... I'm sure you'll do find. Although, personally I'd listen to what 'notepage' just said: you probably have no idea whether the app will sell or not, so the first & easy way to get started would be to try selling the existing software as an affiliate. That way you can try and see how well it sells before you start to pour hours into product development.