- Free
- Lots of subject areas (not just computer sci.)
- People may cite your work
- People may offer feedback
You do need to get endorsed first though [4] (as another comment mentions).
However, if you want "proper" peer reviewed & published, you need to start looking around at conferences. They're usually easier to get something through, as they tend to expect much shorter papers.
There are industry led conferences (like RuhrSec [3] for Cyber Security) which might be a good route to start with.
But, really, there's no shortcut for getting into a "proper" journal / conference. You need to research, have funds, time and, sometimes, a bit of fame already.
[1] https://academia.stackexchange.com/questions/16832/why-uploa...
[2] https://arxiv.org/help/support/faq#1C