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).
Escort quests can take awhile to do. I have thoroughly tested all the escort quests in the game for time/xp. Most are worth doing, but as long as you are able to actually do them without failing. Most require you to pay special attention to the escortee as most are fragile and will die easily, so do your best to keep mobs off of the escortee during escort quests. The longest one in the game is the Tanaris chicken escort. The chicken escorts are only worth doing if you were able to do all 3 of them, as if you do, you will be able to turn in an additional easy quick XP turn in quest at BB, which makes them worth it.
This is the most advanced tip that will only suit the smart players – those willing to spend some time in the auction house. In order to start with the speculations, you’ll need some initial funds and some basic knowledge about the prices on your server, so this tip might not be suitable for many. Back in the day, in order to sell stuff quickly, people used to sell various stuff cheaper than the average price standard on the market and your job is to find those deals and quickly buy them out before anyone else does. This is where the real gold making comes from!
To actually get access to the beta, which has already started and continue to add more players, you need to sign up via your Blizzard account management page. Under Games & Subscriptions, scroll down to Beta Access and visit the Beta Profile Settings page. Once there, you’ll see a grid of available betas you can opt into. Once you’ve checked WoW Classic, hit Update Preferences and you’re set.
Only really to expand on Gráinne's comment as regards the 'collecting' and 'roster development' sections - which are, as said 'distinct' although they are intrinsically linked - I would like to point you in the direction of the in-game Achievements > Battle Pets > Collect tab. There are a series of achievements called Raiding With Leashes, pretty much all of which can be completed via the auction house - if you have the gold/inclination - or by completing old raids.
The original WoW was painful. Mobs took forever to die; one additional enemy in a fight was a pain, two probably meant death. There was a ton of running. Most buffs lasted two minutes, many took reagents, abilities were trained and often out of reach if you lacked the necessary gold. Warlocks had to farm shards, hunters had to carry ammo -- even my warrior did, since in vanilla WoW she can carry a bow and fire arrows herself.
Most profitable professions are considered to be Herbalism and Enchanting in pair. You can have Enchanting only for Disenchanting skill, it will bring decent gold. Consider the fact of disenchanting all soulbound armor, not just selling them to a vendor, but I do not recommend to disenchant any kind of weapons, it is more profitable to sell it to a merchant if it is a soulbound item, or sell it on auction house if it is BoE (bind on equip).
Where to get materials and reagents you may ask? Buy for cheap off farmers in bulk or track Auction House lots. Get in touch with most big sellers. Communication is important here, send a whisper or write them a letter. It would be good for both of you to establish business relationships. Constant suppliers and buyers is convenient for both parties to save the time and effort.