Documentation decays immediately—write it while you're in the code. The habit: comments for why (not what), READMEs for projects, decision records for architectural choices.
Start with code comments answering "why was this done this way?"—future you will thank you. Then project READMEs: what, how to run, how to develop. Keep it tested: if the README is wrong, docs are wrong.
Tools: GitHub wikis, mdBook for internal tool docs, Notion for team knowledge. What gets written gets maintained—write less, write what matters.