Tools – you never know when an axe, shovel or pick might come in handy! Iron is a good choice of material for tools as well as weapons and armor. https://premiumminecraft.com/diamonds-friend-or-foe/
Weapons – the sword is a minimum for basic defense. A bow is equally handy for emergencies and range attacks, although a larger supply of arrows will take some time to obtain.
Armor and shield – both optional, but great protection against enemies or falls. Keep the shield in the off-hand.
Resources – a stack of wood, cobble and dirt will make things easier for you. Keep dirt in the hotbar for quick emergency building. Use the cobble and wood to make a work bench and extra items (or to build) as required.
Food – duh. Always carry a plentiful supply, depending on the food (how many hearts it regenerates) and on how long you’ll be out (though you can gather more on the go). Start a farm early on to keep hunger at bay!
Torches – maybe half stack. Always good to have a supply, just in case.
Others – A map comes in handy, in case that you have one. Consider a bed if going away for long (although sleep will alter your spawn point, so keep track of your bases location).
Part of my concise inventory series: https://premiumminecraft.com/survival-inventory-layout-and-space-management/