Depends on how crazy you want to get with it. Make sure you put down a drop cloth. When doing a large area you want at least a 3inch horse hair brush. Dip your brush in the paint and just spat!
Choose two paint colors: One that will be the main color for your wall and one that is a good accent. Splattering is pretty simple, just don't overdo it.
Splattering paint bedroom wall designs can be as simple as taking the paint of your choice, dipping a brush in it and then flicking the brush at the wall from a few feet away. You will get the random splatter you are looking for with little difficulty.
Very messy - mask off/cover areas you don't want splattered, including floor. Wear gloves. Load paint on toothbrush or stiff paintbrush. Draw thumb or dowel across bristles.