Painting won't hide cracks and holes in the wall. If the wall is uneven, in any way, take care of this first otherwise it will stand out when you are done painting. The paint will even make it shine through more so. If you don't want to highlight those cracks, holes and uneven areas, fix them first.
What type of paint you are using matters for your painting brush selection. Natural bristles should be used only with oil based paints. Synthetic bristles are perfect for water based and latex paints as they will not go limp by soaking the paint up.