Follow professional decorators on Insta/Youtube or wherever and watch their videos on how to do things. Personally having refurbed several houses over the year my favourite is Charlotte Decorator on Insta for good advice.
Think about your whole house, not just each room individually. How many rooms can you see off the hallway? Do you you want them to compliment each other? For me downstairs all complement whereas bedrooms are individual spaces where you can do anything.
Then pick up a paint card ie Little Greene Paint company (they'll post one out to you) those colours are laid out specifically so you can either go across the line of paints and everything in that line all compliment each other or vertically they all compliment each other.
Always get a tester pot, paint it on at least 4 or 5 white A4 sheets of printer paper and stick those up in the room you are going to decorate, especially in the corners by the window. Paint looks different in morning light vs afternoon light and look at it when you turn the lights on, harder at this time of year. Easier to remove those pieces of paper than paint over a million colours you try out on the wall.
Secondly. paint tray liners (plastic inserts you put into the paint tray) are fab, no clean up, just dispose. I would rather do that than waste clean drinking water rinsing out a paint tray.
When you start painting, don't do a whole room, start small, practise either somewhere that will be behind furniture or do one wall. Cut in, paint one wall, cut in again, paint the wall again. Perfect your technique on that one wall. I have done the one wall at a time in my bedroom due to the amount of furniture, easier to move it, paint one wall, move it all back, next weekend do the next wall.
I agree with Toupret wall filler, you will need to mix it up yourself, it comes as a powder and you add water. If you have time, buy a small piece of plasterboard from B&Q, bring it home, deliberately damage it, now try to repair it. That is your practise piece. Buy an electric sander (watch Charlotte) for prepping woodwork and walls. Always wear a dust mask.
Charlotte will tell you what paint covers dark woodwork, what stain blockers to use, what brushes and rollers to use, good quality paint, brushes and rollers give a good finish. She also shows you how to clean them (washing machine) and you can learn a lot from watching Insta/TikTok/Youtube.