It’s in pretty darn good shape for something that hasn’t been decorated for a decade!
Have no idea how much you know or don’t know about prep so will talk to you like you don’t know a lot.
What is the woodwork painted in? Gloss or satinwood? Sand it all back with 120/180 grit paper. When sanding you are meant to work up in grades so if you start with 120, then finish with 180. Again, fill any holes or dings and sand back so flush with the wood. I use a 2 part epoxy wood filler. Hoover up all the dust and use meths/white spirit to wipe over all the woodwork ready for painting. If you are using an oil based paint as your top coat, then I would use white spirit. If you are using a water based paint then I would use methylated spirit.
Most of walls just need a wipe over before painting by the looks of it. But it depends what the paint is as to what you wash it with. It looks like matt emulsion to me, if so use sugar soap or a few drops of fairy liquid in a bucket of warm water to remove any grease, stuck on bits, spiders webs etc. Remove the flaking paint from around the cracks and fill and sand. If there are any dings fill those too and sand. If there are any paints drips, lightly sand those as well. Use fine sanding paper, something like 180 or 240 grit and go easy.
I hate MDF and would never use it so can’t give any tips but this guy can. I’ve watched his videos before and they are good. He’s called the Gosworth Handyman.
Incidentally, that join between the bannister and wall looks like it’s splitting because it is receiving shock from the natural movement of people going up and down the stairs. You might need to dig it out, fill with foam, sand and fill to stop the crack from coming back. Check out the ‘Painting & Decorating’ channel on YouTube, those two guys are professionals.
