Hm, an unusual choice of platform. As for languages, as previously mentioned Assembler, C, or Basic were primarily used for the DOS platform, although there's others like Cobol and Fortan. Each of those languages has their particular advantages and disadvantages, but I would recommend given your level of experience to start with something like [Free Basic], with their documentation and resources [here]. Of course, you may want to challenge yourself with [Assembler], C or others if you choose.

I should point out that Assembler, C, Fortran, Cobol, etc. while difficult conceptually, can be very powerful and do have some uses in the wider programming community, whilst Basic is largely depreciated. This doesn't mean you won't learn anything or that its a wasted effort, but it might be a better idea to position yourself for future growth by starting with a language like Python. Regardless, if you need any assistance with your choice of language, we'll help as best we can.

Dude, just learn python and develop terminal apps. More people will use it anyways.

I wouldn't go freebasic, I'd go C.  Freebasic is C with a weird syntax, in the same way Purebasic is.  Source: Freebasic was my basic variant of choice back when I didn't know enough to realize I was coding in C with a weird syntax or that maybe I shouldn't code in C.  You might as well use the mainstream option here, if you're really going to try to do this.