On all realms including PvP servers, players are safe in their faction-controlled zones (usually low-level starting areas) and major cities. By safe, I mean you aren't flagged for PvP by default. On Normal servers, all players are not flagged for PvP by default. This means that players of the opposing faction cannot attack you unless you toggle PvP on, which you can do by typing '/pvp' sans the quotes. Until you toggle that off by typing '/pvp' again, you will always be flagged no matter where you are. When you toggle PvP off, you will need to stay out of combat with the opposing faction for five minutes before you become unflagged again. If you are flagged for PvP when you enter a zone controlled by your faction or a Sanctuary, the five minute timer to de-flag starts. Players also become de-flagged when taking a Flight Path.
A flagged player will have their faction icon (or in the case of Arena zones, an Arena symbol) beside their portrait, indicating their status. Flagged players will see other flagged players with a red name bar, while unflagged players will see flagged players with a yellow name bar much like a mob that won't aggro unless attacked. Unflagged players will appear with a blue name bar, indicating that they cannot be attacked.
On PvP servers, players can quest and explore without fear for the first twenty or so levels as most quests are confined to faction-controlled zones such as Durotar and the Barrens for Orcs and Trolls or Dun Morogh and Loch Modan for Gnomes and Dwarves. At higher levels, roughly at past 20 and 30, it becomes necessary to enter contested areas to continue leveling.
On Normal servers, a player will always be unflagged and will only be flagged by toggling PvP or attacking a flagged target. Take care not to attack flagged players if you don't wish to engage in PvP. Some players deliberately get in the way of mobs hoping to bait an attack.
Always be mindful of the zone you are in. Some zones, even though Contested, are more friendly to certain factions through sheer proximity to controlled zones. For example, Redridge Mountains and Duskwood are both Contested areas but are relatively safe for Alliance to move through because both zones are Alliance-favored, with no Horde quest hub. Stonetalon Mountains and Desolace, on the other hand, are excellent places for Horde players to quest through with little interference from the Alliance.
That said, on PvP servers there will always be players who move around in an effort to grief lower-level characters. Blizzard makes note of this in their guide to surviving PvP and warns players that it's all fair game on a PvP server. Keep an eye on the General chat channel (Channel 1 by default) to see if players call out gankers and griefers. Subscribing to the LocalDefense or WorldDefense channels also allows you to see if any bases are under attack, so you can choose to either avoid conflict or rush into it head on.
On a PvP server, you will get ganked. That's a guarantee. It comes with the territory so be prepared for it -- be patient and expect to see the Spirit Guide often. Some zones lend itself well to ganking such as the densely populated Stranglethorn Vale and ever-popular Hillsbrad Foothills. You can minimize conflict with the opposing faction by being prudent with your choice of zones as well as playtimes. If you can manage to do most of your quests during off-peak hours, the chances of getting ganked can lessen considerably. Try to avoid timed or escort quests because these are easily disrupted by enemies. If you can manage it, try to move around with a group as there's always strength in numbers. Even a single leveling partner can help as it will make enemies several levels higher think twice about ganking you.
Strength and Honor
Some people may not like spontaneous PvP but are interested in trying it out. For those people, there are Battlegrounds or Arenas, which are controlled environments for PvP. Everyone inside these zones are there for PvP, so it's all fair game. Outdoor PvP objectives are also available for those who seek conflict, although you will encounter these at much higher levels. Players can enter Warsong Gulch as early as Level 10 and Arathi Basin at Level 20. Battlegrounds are a fun and engaging way to practice your PvP. You also earn Honor points from participating in the Battlegrounds, earning you rewards along the way.
Killing enemy players will grant Honor points, provided the enemy is within the proper level range (not gray to you). It is the most basic source of Honor and is also the most volatile, as the amount of Honor gained from killing players vary depending on your level, your opponent's level, how many people were present or in the vicinity of the kill, and how many times you've killed your opponent. Although Blizzard has lifted the diminishing returns on player kills (it used to be that you could only kill a player ten times before rendering them unable to grant Honor when killed), there is still a cap although it is much, much higher. This means you can meet particular players on a Battleground, for example, and continue to reap Honor off them.
You can check on how much Honor you have by accessing your Honor tab (press 'H'); the number at the top is the amount of Honor you have available to spend, while the number on the left side is your estimated Honor for the day. Honor used to be tabulated at roughly between 12mn to 2am every day, but are now calculated in real time. A player can only have a maximum of 75,000 Honor points at any time, so it is always a good idea to purchase items with Honor if you are about to hit the cap so as not to waste Honor points. Participating in PvP Battlegrounds is a good way for lower level characters to get superior-quality gear. Players can also purchase food and drink through Honor points and a few silver, making PvP an alternative means of supporting oneself through the game.
You are not prepared!
Actually, if you rolled on a PvP server, you probably are prepared for the dangers of PvP. On a normal server, you have the advantage of participating in PvP on your own time and on your own terms. When leveling in a PvP server, there are a few handy things to remember -- such as always keeping your life at a healthy level. Back when I used to play on a Normal server, I would grind mobs until I was at a sliver of life before stopping to eat or drink. That would be extremely unwise on a PvP server where a low health bar can turn an otherwise disinterested opponent into an Honor-hungry opportunist. Try to keep your health high as much as possible, to provide a moderate safeguard against enemies who would strike at you while you are engaged with another mob
Try to travel off the beaten path in an effort to avoid other players. While you'll probably encounter more mobs, you're less likely to bump into higher-level players who take the roads as they cross the zone. Remember to always keep your cool and have fun. PvP can be frustrating sometimes, particularly in situations where you're completely -- and unfairly -- outmatched, but those are merely small challenges to leveling. Eventually, you'll hit Level 70, get more practice with PvP, and perhaps choose to inflict the same pain on others. What goes around comes around, after all. Happy hunting!