Wash it reasonably often, especially in winter with salt on the road, dry it properly, then wipe with an oily rag. Touch up any paint chips. Maybe waxoil the inside or use framesaver (it burns, it burns...) I’ve never bothered with either and I haven’t died.
Surface rust is common and removable with a gentle scrub. It’s nothing to be worried about, but can look a bit rubbish. But if you leave it or leave crud (esp. salty road crud) in the nooks and crannies in the frame, then you can be in bother.