Believe or not, Facebook developed a pretty neat software called Origami Studio. I gave some of their tutorials a try and it is actually quite easy to grasp and use. I’d take a look, at least out of curiosity.
Considering the other comments and your replies: Do you have any friends who have a Mac and are willing to help?
You still need to know the language and how to actually develop an app... and tons of free stuff online. Hacking With Swift (already suggested) is a great place to start...
Also, and this might seem strange, but do you want an actual, fully fledged app that people can download? (Then as mentioned, you need a paid developer account, it's yearly)...
If not though, if you want it free just for the look and interaction, you can use a prototyping app... something like Origami Studio, not sure if there's one for Windows... but you can check any free prototyping software... so you basically get a mock-up that creates the illusion of an app... because an actual working app, you already have the answers for that.
I mainly use Sketch and Adobe, but unfortunately, you're right, there's nothing cost friendly about either. I'd recommend trying Origami Studio.
Are you planning to become a UX Designer after your degree?