Depends on what exactly you want to do
For your classical "fat" client, go with Lazarus (Open Source Delphi clone) which will happily compile on just about any platform.
As for SQL, stay clear of MS-SQL, it's probably the worst solution out there. MY-SQL works just as well, and when you have some small solution you can even get sqlite which is like an SQL-server, but without the extra program. SQL is good for anything you need indexes for.
If you don't use indexes for a table consider plain text files. Those may be a great solution for things like "user lists" and they give you a simple way of getting your system integrated with other systems.
However today you should be looking into HTML5 or text terminals for the user interface. Ideally your package has a text interface between the user interface and the logic. That way you can bolt on any kind of user interface you like.
In any case read "The Art of UNIX Programming" (available online) and understand it. It contains important ideas of how to make any modern working real life system.
And please stay clear of the C# crowd. That is, when you look at the whole picture, one of the worst things you could be going to. It's non portable (won't even run on all common versions of Windows) and only supported by one single vendor with a good track record for abandoning projects even though people are using them.
Biting the hand that feeds IT
