'Cut in' first, with a decent sized brush, painting the corners, edges and around plug sockets, light switches etc until all that's left can be rolled.
Don't overload the roller - use the slope on the tray to evenly spread the paint around the whole roller before applying it to the wall.
Get a telescopic handle, it makes it much easier to see where you've been and that you're getting an even coat.
Only roll vertically, overlapping slightly as you go.
HTH, I worked as a P&D for a year or so and you'll soon get the hang of it.
