The only way to get a good finish to any paint work is how you prepare for the painting.
If you have any signs of damp stains or mould stains wash them with a solution of bleach and water mixed 50/50 with bad stains you may have to do this twice.
Remove any screws or nails in the walls before you start painting.
Fill any holes or repair any damage with polyfilla and smooth the damaged area down level.
Treat or wash any stains on paintwork before you start , heavy stains will only come through the new paint.
Sand and smooth old paintwork , doing this will make sure you get a good finished job.
Clean all surfaces with sugar soap this will remove any grease etc , which people have left behind by touching the paintwork such as doors.
But you may not have to do all or any of the above as your paint work could by fine.
Make sure you have all the correct brushes , rollers , steps , and any other tools you think you may need.
Now you are finally ready to start , all you have to do now is go out and buy the paint.