Are you thinking only of building apps for desktop, or might you build a Web app (ie like a website, which runs in the browser)?
If the latter, then you will need to learn a mixture of HTML/CSS and JavaScript, and probably add in a web framework such as ReactJS or AngularJS (there are quite a few others).
You can use other languages to build web apps but I think it would be easier to stick with the above unless you want to build desktop apps, which run directly on Windows or Mac or Linux or iPhone or Android. Note the "or"s as it is not easy to make a desktop app work on different operating systems, and mobile is really more like another app altogether.
That’s why I favour Web apps where suitable, they will pretty much work on anything with a SAFE browser, which will be all of the above. You should need to do a fair amount of tuning to work well on all different sizes of devices - screen sizes that is - but it is much easier than building for different operating systems.
So if you go for a Web app it is much less of an issue which language to learn - start with JavaScript because there are lots of resources to help you learn, lots of libraries ready for you to use, and already many app and code examples already running on SAFEnetwork available for you to learn from.
That’s important, because one of the best ways to learn is by examining other people’s code and then trying to adapt it, or even just make superficial changes until you are ready to build an app from scratch.
A Simple SAFE Website
If that route sounds good, start with a simple website and get that running on SAFEnetwork. The following topic should help with that:
Then you can add to this, start with getting your website to authorise on SAFEnetwork for example, and then add to it step by step.
Or you could begin to look at code that others have produced to do similar things and understand how that works. There are posts on the forum about such apps but some are quite old and may not have been updated so I won’t post links here in case they send you on a wild goose chase!
Ask here when/if you want some tips and people will help out.
SAFE Apps
If you want more on options for development I wrote the following post a while ago, but this isn’t really for beginners. It is quite old and some details will have changed but I think it is still a reasonable guide.