Does anyone have any ideas about the best dictionary for a beginner?
Right here, to start with: Gerlyver Kernewek
That’s the online version of A Learners’ Cornish Dictionary (2nd Edition), which is the most comprehensive SWF dictionary available. It also has plenty of useful grammar notes for beginners, which I don’t think are included in the online dictionary at that first link, so it’s worth buying the actual book.