How to get started painting a big room?

Start at the beginning of a wall & work your way around

move furniture into middle of room, remove drapes and dust off walls patching any holes and removing and nails or hooks. Don't forget the dropcloth!! I do one wall at a time, taping if you have to, I just cut in along the trim and window and door frames then prime that wall and repeat til done. Second coat is easier because it is your choice of paint color. Sometimes you may have to give it s second coat depending on the coverage. I personally like painting and love to redo my colors every 2-3 years.

Move as many things out of the room as you can what you can't move push to one corner of the room and cover with a drop cloth. Patch any holes that are in the walls and tape off what you don't want to get paint on.

Wash the walls with TSP, let dry. Then tape off areas not to be painted. 1 or 2 coats of a good primer, letting dry between coats, then a couple coats of paint. Then remove painters tape.

Cover or remove any furniture in the room first and then tape around any woodwork such as windows.

https://www.youtube.com/watch?v=us1yHytgOLY

I'd like to say "just start" but that seems too simple because it's a lot of work. Are you covering paint with paint? Go to a hardwear store to get the following: floor cloth, paint brushes, roller and good paint. Do not skimp on these products because you get what you pay for. If you aren't painting the woodwork tape it for with painters tape. I can't Stress enough how important it is to always paint in the same direction so you won't get assorted brush strokes.

start at the top (ceiling) work your way down