Minecraft wiki entry for The_third_day, associated text: A bed: these reset your spawn point and skip the whole night without having to fend off monsters. Sleeping also fends off phantoms for three days.
Weapons
A shield
Iron Armor: at least 1 piece, preferably the chestplate
(Stone/Iron/Diamond) Pickaxe
(Stone/Iron/Diamond) Shovel
Cooked Food
Torches: Lighting up your house prevents monsters from spawning in your house, especially at nighttime.
Place any unnecessary items in a chest. Any items placed in a chest will not be lost if you die.
[Optional] Clock: If you are in an underground shelter due to having no time to create a shelter with windows and doors, you can use a clock to see what time it is outside without opening a hole, risking a monster spotting you.
[Optional] Compass: This tool can help you find your way back to the spawn point.
[Optional] Map: This shows a top-down view of the area and can be more useful to find your way home than a compass.
[Optional] Empty Bucket or 2-4 Milk buckets: Milk Buckets will help you negate any effects inflicted on you, which is extremely useful if you encounter a witch or a cave spider which gives you poison. For Beginners: to obtain milk buckets, just go up to a cow and use an empty bucket on it and the bucket will have milk in it; this milk can then be drunk to negate any status effects.