First Aid is one of three secondary professions in Classic, alone with Fishing and Cooking. Being a secondary profession means that it can be learnt in addition to two primary professions, which include professions such as Herbalism and Alchemy. Just like other professions, the maximum First Aid skill is 300. First Aid enables players to craft and heal themselves with bandages, and craft anti-venom to remove poison effects. These items can also be used on other players, as well as some NPCs. Bandages can be crafted from cloth ( Silk Cloth, Mageweave Cloth, etc) that drops from most humanoids, with higher quality cloth producing more effective bandages. First Aid will be one of the earliest professions you pick up, as the self-heals provided while leveling are very useful, especially if you play a class that does not heal. Anti-venom consumables can be crafted from venom sacs that drop from venomous monsters ( Large Venom Sac, Huge Venom Sac, etc) such as scorpids, spiders, and most insectoid creatures.
Benefits of First Aid in Classic WoW:
First Aid helps reduce downtime between battles, as bandages will quickly restore health post-combat. Classes with healing spells will also benefit from First Aid, as using a bandage will help conserve mana. Hunter and Warlock players can also apply bandages to their pets and demons respectively.
In PvP encounters, players can use bandages to restore their health while in combat or provide triage to other injured players that are not actively taking damage.
First Aid helps with conserving healers' mana and casts in dungeon, raid and PvP environments, as bandages will allow players to heal themselves and each other while they're not actively taking damage.
Characters below level 35 can raise their First Aid skill to 225 to use Heavy Runecloth Bandage, provided another character creates the bandages. This is primarily important for players that create twink characters in this level range (1, 19, 29, etc). This can also help with leveling alt characters, as they would have access to much more powerful bandages.
Learning and leveling all of the secondary professions is highly recommended for all players (and considered mandatory for serious raiding and PvP), as they provide additional utility and alternative methods to earn gold. For the other secondary professions, consider checking out the Cooking and Fishing guides written by Quissy.
As First Aid was removed in Battle for Azeroth, you may need a refresher on how bandages work:
Bandages can only be applied to the same player/NPC once every 60 seconds, as bandages apply the Recently Bandaged debuff (similar to Weakened Soul).
If the player applying the bandage or the player/NPC being bandaged takes damage, the healing effect will be canceled and the Recently Bandaged debuff will still be applied.
The player that is applying the bandage must not move, otherwise, the channeling effect will be canceled. However, the player/NPC that is being bandaged is free to move so long as they remain within range.
Bandages have a range of 15 yards when used on other players/NPCs.
Anti-venom consumables function like a standard consumable with a 60-second cooldown. (ex. Strong Anti-Venom). Anti-venom is important to have for PvP, dungeon and raid boss encounters - for example, using Powerful Anti-Venom to remove the Brood Affliction: Green debuff in the Chromaggus encounter in Blackwing Lair, or to remove an enemy Rogue's Crippling Poison. Anti-venom is also beneficial in that it effectively frees up another cast for healers to utilize.
Here you can buy Classic First Aid Leveling for EU or US Servers.
You'll get guide for completing Classic First Aid leveling