Further to my previous post on getting started with Debian. Once you are up and running. If you would like to help with development, there seems to be a multitude of options in terms of programming languages.
In terms of Kernel development and CLI tool development then C seems is the primary option, you should therefore. You should therefore perhaps take a look at https://gcc.gnu.org/ to get started. It would probably be assumed you are familiar with C anyway.
In terms of other languages then
would also be a good starting point. Depending. In terms of application development then is is probably dictated by which ever graphical interface you are using, as gnome and kde would use different libraries to make the applications buttons, menus etc.
Again best place to ask is on the mailing list, once you have joined the Debian-users list you can be signposted to other lists from there.
Of course you can also ask on IRC via OFTC
Freenode has channels relation to some of the above programming languages.
This list and information is not exhaustive but I hope this can provide a general starting point.
Update 26 May 2019
I have added a poster with basic information on helping the project.
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