Hello Lily!, welcome to the forum.
I'm guessing that you have ArcaOS, right?
I think you first need to read this to understand the tools that we have:
https://www.arcanoae.com/wiki/information-for-developers/Our resource for development articles and API information is the EDM/2 wiki.
https://www.edm2.com/Where to start depends on the goal of your project and your skill set.
a) If you want to go pure Presentation Manager (PM) you should use OpenWatcom (open source) and/or IBM OS/2 Developer’s Toolkit (OS2TK45) (Freeware) and/or IBM VisualAge C/C++ (Not freeware/Abandonware)
I did some experiment to compile old PM samples with gcc. I was told that OpenWatcom is better than gcc for pure PM apps, but it was just an experiment since with gcc it was very easy to install the environment with rpm/yum
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https://github.com/orgs/OS2World/repositories?q=DEV-SAMPLES-PMb) If you want to port things from the Linux word you can use "gcc" and some libraries like "Qt5" and SDL.
c) Do you want to create a WPS application.... prepare to suffer

Maybe you should start with PM first and later learn about WPS.
d) If you want to develop drivers you require a different set of skills and tools like IBM Device Driver Kit for OS/2 (IBMDDK) and OpenWatcom.
So, there are several options according to the need. Just read and think about what you like to start with, and just ask away on this forum.
Regards