[phba]

A good place to start is OSINT (open source intelligence) for your city/municipality because it requires little commitment, is scoped with regards to complexity and amount of information, and usually risk-free. Gather publicly available information about the companies in your area, who owns/runs them, your city council, any ongoing projects, the processes of funding stuff with public money and so on. Don't bother finding the best collection method or way to structure all the data, just start, you will figure things out on the way. Also be aware of your personal bias, which might make you dismiss important information or affect your judgement.

The next steps highly depend on where you live. Your HN profile says Australia, so at least safety-wise you are in a better spot. Connect to people in your area (preferrably offline), for example by organizing a local meetup, maybe there is one already. Activities can range from exchanging ideas to spreading awareness in your community to actively going against corrupt affairs. Make sure you know what and who you are up against, or you will have a very bad time.

Anticorruption is a group effort because it requires a lot of work and often special knowledge (info tech, law, finance, opsec, public relations and propaganda, ...) and, more importantly, a group provides safety from corrupt actors. On your own you will not be able to deal with lawsuits, misinformation, character assassination and worse.