Minecraft wiki entry for Hardcore_mode, associated text: Nether wart - they are necessary to brew any useful potion. They are found quite often in nether fortresses. Five is enough, but it's better to take every one. Collect at least six blocks of soul sand, so that a farm can be started.
Blaze Rod - it is necessary to craft eyes of ender. It is also used to craft a brewing stand and brew strength and fire resistance potions. To obtain them you will need to kill blazes. It's best to fight lone blazes at first to avoid becoming overwhelmed. Find two rods (one if you already have magma cream); once you have those (and your Nether Wart), retreat to brew some potions of fire resistance. Collect more rods after applying the fire resistance potions, but remember that potions of fire resistance do NOT nullify Blaze melee attacks. For the main Blaze battle, find an enclosed spawner and kill them with a sword. This conserves arrows and saves the drops. About 12 blaze rods should be more than enough.
Ghast tears - used for potions of regeneration. However, ghasts are dangerous, and their drops are often destroyed.
Glowstone dust - used for strengthening potion effects. Glowstone appears occasionally in nether fortresses, otherwise, look for it on low ceilings of netherrack. Besides, it allows you to create spectral arrows, the effect is just a visual help, but every creation it means +1 free arrow in exchange of glowstone dust. Keep in mind that spectral arrows are consumed even if the bow is enchanted with Infinity.