I did not attend university, I left school 4 years ago and and I am not affiliated with any but I have an extremely useful compression program which I've created and I want to publish it but have little idea on what to do next. Any help would be appreciated.
Like anyone else, you can write a scientific paper describing it and submit to an appropriate journal. Before you do so, however, explore the candidate journal a bit to see what they publish and how those papers are presented.
The editors and reviewers at the journal will give you additional advice on suitability of your work.
In computer science, annual conferences are also a good venue. Actually they are the preferred venue for most work. ACM, IEEE, and others offer such conferences.
A paper can point to a repository in which you have stored the code if that is appropriate to your purposes.
You don't need an affiliation to do this. "Independent Researcher" is fine as an affiliation.