To get a good result start with a good surface. This means filling cracks, cleaning off stains and most importantly, rubbing down to get a smooth surface.
Don't underestimate the importance of that one piece of advice. In order to do it properly, you have to spend time preparing the surface. Use a sander (or sandpaper wrapped roung a block of cork) and rub every inch of the wall. It is boring, tiring work, but the results really do speak for themselves.
Next, wipe off all the dust with a damp cloth. In all aspects of painting and decorating it pays to be clean.
Now apply the paint using a BRUSH. Rollers give a mottled finish while a well brushed wall will look smooth and professional. A four inch paintbrush is fine for an interior wall.
Start around eye-level, painting horizontally and then vertically. Finish with an almost dry brush in short gentle flicks, to 'lay-off' an even finish.