The first and most important thing is tissue papers because they are one which will be used not only when you would be applying makeup but also when you will be removing it as well.
Cotton buds are next in the list because they will be used, whenever you want to rectify lipstick or eye makeup errors.
Then you must have cotton in your kit because it will help you in cleansing and toning. You need to have at least 5 brushes of different sizes in your kit, one big brush for blush on, two small and thin brushes for eye makeup, brush for eye makeup mixing and one for application of eye makeup and in last you need to have one brush for making of thin lining of lipstick across your lips.
In last I would suggest you to keep your makeup kit neat and clean, so that whenever you need your kit, it is always ready for you.