Honestly... board, trucks, wheels cost us about $100 all told. And it was COMPLETELY worth it. Excluding the helmet (a skate helmet) the BOARD is the "must have". You really, really, get what you pay for. You don't need to drop 500 on crazy bearings, or $50 on wheels... but buying a quality board/trucks/wheels (typically sold 'together' at a skate shop for about $100) is THE #1 for safety and fun.
A skate helmet is important (and we ONLY use the now for biking, razoring, etc) because the shape prevents them from breaking their neck (bike helmets protect the "over the handle bars" hits... but if they're caught on the side they yank the neck) which skate helmets are rounded (and cover more) so landing on your head or banging it doesn't do serious damage to your neck. Skate helmets ALSO cover a FAR larger area (not just the already thick top of your skull, but the thinner and more easily damaged sides, as well as the whole brain stem).
EXTRAS (after the $100 board -or less bought used, and boards last for upwards of 10-20 years- and helmet) would be elbow and knee pads