Get a good brand so it's nice and thick and covers well - Matt or Silk will work, I reckon silk is best for walls, matt for ceilings.
Cover furniture an carpets so you don't get paint speckles on everything from the roller.
Do the edges (and round any lights) carefully with a brush to about 3" out
Do the rest with a roller with a broom stick shoved up the handle - then you can do most of it stood on the floor.
Don't roller too fast or you'll throw off lots of droplets (speckles)
Keep the edge of the paint wet - if you stop half way through for lunch, the edge will dry and will show.
A damp cloth will get paint off the walls, whatever where you touch as long as it's not dried.
Gloss painting.
The undercoat does most of the work killing the old colour - so don't skip it.
I use foam gloss rollers for large areas but it's handy to have a 1 1/2" bush handy for fiddly bits. Lightly load the brush, do the fiddly bits then roller in the big areas. The roller may leave bubbles in the paint, if it does go over with the brush (almost dry) really lightly to pop the bubbles. Be careful not to put too much paint on or it will run.
Ceiling first, Gloss second, walls last
Don't forget your prep, get everything clean, fill in holes with decorators filler.