It depends on what you're trying to run from D8 and D9. If you take a look at Configuration.h, you'll see a section where you select the board. Board numbers 33, 34 and 35 are where you choose what you're going to do with the outputs. Unless you're running two extruders, use 33.
If you don't have thermistors connected, you won't be able to drive the heated bed, as Marlin protections to stop it overheating and burning out. Look further down in Configuration.h to select the correct thermistor types for your setup. You'll typically be setting TEMP_SENSOR_0 for the hotend and TEMP_SENSOR_BED for the heated bed.
Unless you have two extruders, you typically use D9 to drive a fan you use to cool the extrusion for PLA (not used for ABS). If you have an all-metal hotend, this will have a fan used to cool the upper section and stop heat creeping up the hot end. This fan should be always on, so should be directly connected to your 12V power supply.
[
3DKarma.com] - suppliers of quality, affordable 3D printer kits and filament for the UK market.