A new quest will pop up once your first pet reaches level 3. It will send you to either Audrey Burnhep in Stormwind or Varzok in Orgrimmar. These trainers will offer a quest chain to battle specific Pet Tamer NPCs on each continent. Defeating each of these NPCs will reward a nice chunk of experience for your pets, as well a Sack of Pet Supplies (which has the chance to drop the Porcupette companion, among the always-useful pet bandages).
To get Blacksmithing to level 300 is much effort. It is not that thing to make fast gold. There is always a lack of Blacksmiths though. Enchanters need rods of all kinds, melee classes look for Sharpening Stones and Weightstones with Counterweights, others buy Iron Spurs. Once you have learnt rare and epic recipes, people will always use your service to craft items.
Through out the game, the quest log will be capped to 20 many times. The guide makes use of maximizing the total amount of quests you can accept at once. Because of this it is important to follow the guide exactly and only accept quests if the guide tells you to so you won't run into issues not being able to accept a quest. If you do accept a quest that is not listed in the guide, write it down because you may need to abandon it in order to keep following the guide if your quest log is full.
Swim Speed Potions (used especially at lvl 37 for Faldir's Cove in Arathi Highlands and lvl 57 in WPL). Try to get multiple stacks of these. I also recommend keeping a stack of these always in your bags. You will frequently pass rivers and lakes and can use these potions to pass them faster through out the guide. Please note that these potions do not stack with Druid's Aquatic Form and Glyph of Aquatic Form.
In terms of what the World of Warcraft Classic Best Classes are, Warrior and Priest stand out almost instantly. This is partly due to the fact that Protection Warrior is the only viable Tank Class in WoW Vanilla Patch 1.12. Both the Proc Warrior and Priest are required for Raids and when combined in a PVP setting can be a fierce combo on the battlegrounds.
Things like Tauren melee reach being bigger, which is something I experienced going away live during Black Temple Illidan progression (at least for that particular encounter), is confusing to players as it's a significant advantage over other races in some instances. No quest tracking is another apparently confusing issue, as many Mankirk's WIfe seekers well know, and there's a whole lot more where that came from: