If you found this guide without knowing what twinking is, I'm very surprised!

Twinking can mean many things, but in World of Warcraft it refers to making a low level character as powerful as possible for various purposes. Twinks will often turn off their experience gain to keep their toons at a certain level as long as possible, but this does affect other aspects of gameplay. 

This guide is aimed at those who want to disable their experience gain at levels 10-11 (and I guess up to level 14, but some suggestions will not be suitable at higher levels) for the purpose of Player vs. Environment.

At this moment of time, 10-11 are the earliest levels where you can participate in a wide variety of game play (many quests, professions, dungeons, zones, etc.). Certain races also start around level 10, so characters created with these races can immediately turn off their experience and start gearing. 

Due to level scaling, 10s can often outperform higher level characters in damage and healing. This makes them ideal for boosting, farming, specific niche purposes (e.g. healing To the Test: Symbiotic Glowspore Grip). Level 10-11 is a great level bracket for speedrunning, glitch hunters, and those looking for novelty.

When most people think of World of Warcraft twinking, the image that comes to mind is a level 19 rogue, ganking in battlegrounds. However, this image is a far cry from the twinking brackets of Retail. When a player's experience is disabled, they can only do battlegrounds with other players with disabled experiences. Therefore, PvP for XP OFF 10s would be duels and possibly wargames if there's enough organization. Otherwise, the 10s bracket is primarily PvE.

No! While obtaining certain best in slot items and enchantments may require a paid account, these pieces are more bonuses instead of requirements. There are substitutes for many of the suggestions given. Plenty of people are able to and do play low level twinks on starter accounts, and have a lot of fun with it too. Just know you will be going in with a lot of restrictions, and that might be a lot less fun for you!

XPOff.com (all)
Legacycraft (10-20 PVE)
Golden Twinking (20s PVP, 30-40 PVE)
Legacy Thirty (30s PVE)
Wuck Falls (70s+ PVE/skips)