Minecraft wiki entry for Surviving_in_a_single_area_indefinitely, associated text: 1 or 2 buckets, which require you to find iron (by mining or raiding a weaponsmith's chest) to craft
at least one sapling of some type (except dark oak saplings, in which case you need 4)
access to at least one source of lava and water (a village with a weaponsmith shop has a lava pool and water from village farm or fisher cottage)
melon or wheat seeds