First, make sure you log into all of your characters and sit on them for a few minutes. This will let all of the account-wide stuff populate into your collections. (Transmog particularly can take a few minutes the first time.)
At level 35, you can buy adventuring gear from Mists of Pandaria vendors, which will be the easiest way to get some level appropriate gear. (Various changes to the game mean that characters in old end game gear are often comically undergeared for their level.) The easiest vendor to get to will be Singegruff in Dawn Blossom in the Jade Forest. (Take the portal in the portal room to the Jade Forest.)
Next, I’d see if you have access to Chromie Time. The official requirement is getting a character to level 60, but I’ve seen people with characters that were max level in previous expansions who have access. (I’ve also seen people say you can meet the requirement by creating a trial character.) Chromie is near the Embassies in Orgrimmar and Stormwind, and is marked on the map by an hourglass icon. The Orgrimmar Embassy is in the Goblin part of Orgrimmar (which was the Troll part pre-Cataclysm.) In Stormwind, it’s strait out the back gate in the Dwarven District.
Turning on Chromie Time for any expansion will cause all old expansions to scale up to 60. The specific one you choose will determine which dungeons you see in the dungeon finder. So, if you wanted to do some more familiar dungeons to get back into the swing of things, you can turn on Chromie Time for Wrath. (But, you can still quest in any expansion even when it’s set to Wrath.)
For specs, you can just start off with the starter build option the game has. They aren’t perfect, but their fine for leveling. For more in depth info, Icy-veins.com has guides, as does Wowhead.com. (Many classes also have discord servers for the real heavy theorycrafting.)