Welcome! Do you have any familiarity with using Octave? In the Octave wiki under
https://wiki.octave.org/Summer_of_Code_-_Getting_Started#What_you_should_know_about_Octave there are links (admittedly poorly identified) regarding development and how to clone the source code using Mercurial. There are some links to developer guidelines as well. We use the savannah bug tracker for discussing and submitting specific code patches, as discussed in some of those links. Some ways to get started contributing can be finding an outstanding bug and giving it a try to find a solution. Recommend posting a comment to an existing bug so people know you're working on it, since we don't explicitly assign bugs for people to work on.