First off, if you like your lock, STICK to your lock, yeah sure you can reroll but then thats just a complete waste of time if you ask me, unless your using your lock to gather mats (herbs etc)...
If you like the look of another class then research into it, find out the pros and cons etc...just make sure you enjoy the class your playing
Well like most classes in wow, you'll suceed if you know what each and every spell in your arsenal does, when to use them and how...what skill best goes with another skill etc etc like...deathcoil ---> fear
Demo doesnt really do much in pve, it can be used in pvp if you have a clue on how to use your pet effectively like, utilising the your pet's intercept into a chain cc like fear or following a deathcoil, but that'll come later
Affliction and maybe even destruction now are your pve talent trees, destruction is pretty hard to get used to as you got casted skills and locks can be quite easy to lock out in pvp
The basics are keybinding all your skills, or your most frequently used skills near WASD... dont be a keyboard turner cause that'll get you nowhere in wow :<...turn with your mouse and use WASD to move...have important skils like deathcoil or healthflunnel on easy access keys like F or G
next i'd pretty much say get used to your keybindings :P, go join a bg and pvp around, mess around check what skills go with what skills and mess around
Im mostly talking about pvp because getting used to pvp and how game mechanics work with other classes means you know exactly how different your very own class is...locks are the only class that can fear without having a cooldown, apply the most dots than any other class, have the ability to create healthstones, can literally never run out of mana if you have a healer near you etc etc
Not to mention that the honor reward items are easy to get...just an honor grind and they're good starting items for new toons
Only other thing i can tell you is to watch pvp movies or pve movies...pvp movies will show u things that you prolly havent seen before, great lock i've seen over my time is drakedog however his usually destro...so yeah, scope around the forums and check out other peoples armory's, check out their specs their gear what they usually stack (mostly spellpower for locks)
Erm apart from that, i'd say try n get full blue items, noones gonna want you if your in greens unless your experienced and you know what your doing =P
good sites like
http://www.wow-loot.com/warlock.htm can tell u what instances drop what kinda items for you and what you should roll for if they do drop...try not to need on everything thats blue =p
If your gonna start out pvping then i suggest you stay demo as soullink and your pet will help you out in places, it'll get fraustrating as your gonna meet rogues and paladins that are jus gonna chop away at your hp with 5k hits and what not but its not like you loose anything from going into bgs, coming out with honor and experience :bounce:
You'll get easy kills if you just know how to play effectively, like dotting rogues up, knowing when a guy in bg has used his pvp trinket so you can jus fear him...KITE melee etc etc...
oh and dont have your pet on aggressive...keep it on either defensive or passive, make a keybinding initiating him to attack/intercept...will help alot and wont cause your whole party to wipe in instances ;d