What are the main stats?
The main stat for levelling as a cat is Agility, and you can't go wrong upgrading to any piece with more agility, whatever the other stats may be. At higher levels Armor Penetration, Hit (up to 262 at level 80) and Expertise (up to 16 at level 80) are also valuable, though Agility remains the most important.
What class of weapon should I use?
You should make sure you use a 2-handed weapon, either a staff, 2-handed mace, or polearm. Until around level 55, you should look at the weapon's damage per second as this will govern how much base damage you will do. After 55 you'll see a stat called Feral Attack Power (FAP) and you should use this rather than dps to determine what weapons are upgrades. Look particularly for weapons with Agility ("hunter" polearms are ideal).
Some excellent weapons to seek out while levelling include:
From world drops:
Goblin Power Shovel
If you have access to Heirlooms, use:
Repurposed Lava Dredger
Will I be viable to raids later on?
With decent gear feral dps are very viable in raids, bringing high dps and welcome raid buffs like Leader of the Pack, Mangle, Faerie Fire (Feral) and Gift of the Wild, as well as the ability to resurrect other players in combat, and restore a caster's mana.
What AoE does a feral dps have?
Swipe (Cat) is the AoE ability for feral dps, though Rank 1 is not available until level 71. It is a very powerful AoE - one of the best in the game. Until then, however, if AoE is needed your only option is to switch out of cat form and cast Hurricane.