Until a certain point it is all easy to do by yourself, like servicing the derailleur and cleaning/oiling/changing the chain. No special tools required (a chain opener could come in handy though - a few franks on amazon). That is basically all you need to do on a regular basis (maybe thightening quickly all the bolts with a standard allen key).
But when you start to change the cassette and have to grease the ball bearings special tools and an appropriate workspace is required…
(however, the typical city commuter needs that kind of maintenance only every 2-5 years - so still dirt cheap - about 200-250 CHF for the full service with no parts).