Before you register, you need to choose one of the Sides, you can do that by clicking one of the colored orbs (Light Blue/Red) and then "Create Your Account Now". The only true difference of the sides are that you can only join the clan which has the same side as you.
Protectors of Light Champions are famous for being dominant and just. For them the most important thing is Peace.
Legion of the Damned Champions are famous for being ferocious and rigid. For them the most important thing is Legacy.

Each side has 6 different classes. Classes mostly provide small Experience bonuses to certain activities. Don't worry too much which class to choose, you will be able to change it if you wish. Class bonuses are as follows:
⚔ Warrior/Assassin: +5% Combat Experience (Fighting Fields), +10% Slayer Experience and +10% Gold in Fighting Fields.
⚔ Paladin/Barbarian: +5% Combat Experience (Dungeons/Summoning/Rifts), +10% Summoning Experience and +10% luck bonus to find Riftstone (Dungeons/Summoning).
⚔ Mage: +10% Magic Experience, +20% Magic Experience while using Combat Spells and 10 minute earlier Witches cauldron.
⚒ Guardian/Cleric: +10% Mining Experience, +10% Blacksmithing Experience, +10% Jewelcrafting Experience and +10% Exploration Experience.
⚒ Ranger/Rogue: +10% Fishing Experience, +10% Cooking Experience, +10% Hunter Experience and +10% Crafting Experience.
⚒ Elf/Dark Elf: +10% Herbalism Experience, +10% Alchemy Experience, +10% Woodworking Experience and +10% Beastmastery Experience.

Level Points can be acquired by leveling up your Combat level. At Combat level 1+ you gain 3 level points per combat level, at Combat level 10+ you gain 4 level points, at Combat level 50+ you gain 5 level points, at Combat level 100+ you gain 6 level points. There are 2 Statistics that can be leveled up with Level Points:
Attack - Attack increases the damage you deal. You gain 1 points of Attack per 1 point spent;
Defence - Defence reduces damage against monster attack. You gain 1 points of Attack per 1 point spent;
Base stats are: 250 health, 5 attack and 5 defence. All of these stats can be increased with equipment. Additionally your Attack and Defence can be increased with Charms, Clan Buildings and Pets.

Health is keeping you alive and in order to Fight monsters you need Stamina. Both will recover slowly over time, but you can boost your stamina and health by eating cooked meat and fish, respectively. You start the game with 250 Health and 500 Stamina. To increase your max Health you need to level up your Combat level, you gain 5 Health for each level. For Stamina, you need to increase your Slayer level; with every Slayer level, your max stamina increases by 5 points. Regeneration rate is always the same. For Health it is 10 Health in 10 minutes. If you have Berserk State active, you regain 10 stamina points every 1 minute and 30 seconds. When you're not berserk, you regain 10 stamina every 3 minutes.

Same as your Character, Monsters have attack/defence/health stats. These Stats are listed in the player Wiki page and in the game itself Tavern -> Monster Wiki. Currently there are 4 places to fight monsters (Excluding Events/Special Monsters):
Fighting Fields - The Fighting Fields is the main area where you fight normal monsters to gain Experience, Gold, Gems(1/100), Goblet Of Death(1/200), Ancient Fossil (1/25,000) and Equipment. Each fight costs two stamina, and you will continue fighting battles until you run out of stamina or health. Fight have a max limit of 15 rounds, if you don't kill the monster by then you run away not gaining any rewards. Each field has seven different monsters of increasing strength. Make sure your stats are sufficient to kill the monster and use Fish to restore your health. Fighting Fields are often shortened to FF1, FF2, FF3, etc. in game chat;
Dungeons - Dungeons are areas where you can fight mobs before running into a boss monster. The bosses can drop Set items (Defence based). Fights here will cost a set amount of stamina to enter the dungeon, but no more while you fight mobs inside. You can't heal with fish while in dungeons, only with potion. If you're unable to defeat the boss, you can abandon the dungeon to avoid the gold penalty of death by returning to the dungeon page through the home screen.
Summoning Profession - is a profession that uses one Goblet Of Death and a set amount of stamina and health to summon a boss to kill. The boss rewards combat experience, gold, and Bonus XP. Summoning bosses can also drop Set items (Attack based);
Rifts - Rifts are special encounters against Dungeons and Summoning bosses that give increased gold, combat Experience, and Bonus XP rewards as well as a giving an increased drop rate for equipment. To open a rift you will need to have a Riftstone of the appropriate tier (For instance: Riftstone 1 can be used to open a rift in Spider Cavern, Forgotten Grove or Wardens of the Forest). Rift bosses have the same attack and defence as the standard summoning and dungeon bosses do, but the health is 10x higher. Rift bosses will always deal damage no matter what your stats are and frequently inflict critical strikes;