Hey, I have just started my first Warlock character and was wondering if you guys could give me some help?
 
First of all, I am lvl 18 but have never used any talent points since I have no idea which to choose. Which do you recommend?
 
What aspects are most important on a Warlock? (Spirit, Agility etc)
 
What is the difference between a summon mount and your class mount (mine is a blood elf).
 
Thanks for anyone who could give me a hand.

It's all preference but most of my warlock friends advice against leveling as destruction since it is hard on resources. Demonology is popular but I always preferred Affliction as you can literally just DoT mobs up and spam your wand on them until they die. It's amazing for killing large groups of foes at once. 
 
For leveling stat wise int and spirit are indeed nice, but make sure you get some stamina for lasting longer unless you are really good at avoiding damage thanks to your pet or killing fast.  
 
As for mounts like Shirogane said you really dont have to pay for one. Essentially Paladins and Warlocks can talk to their class trainer and learn how to ride mounts for next to no cost. The only difference in the mounts is visually. All warlocks have flaming horses while the normal Blood Elf mounts are Chocobo wannabes.

Wowwiki is your friend. But from personal memories, affliction is the levelling tree. Being able to instantly apply dots and self heal with drain life, while your voidy takes the hits is the strategy to go for.
 
The most important stats for a levelling warlock are Int (Higher mana is needed as a spellcaster) and Sta (Warlock mana regen is based around that move they have. It drains your HP to restore mana. Name escapes me).  Everything else is second until you get to around 60 or so. Demonology might be good at 60 due to Felguard, but I haven't played a lock for ages.
 
Mount changes are aesthetic. The class one is essentially free, so that's nice. As someone who's ruined their life with WoW, drop me a line if you need any advice, I'm sure I can be of further assistance should you need it.

I've played a lock for a few years and it's my favorite class.  Destruction is difficult to level with, it's demanding on your mana pool and requires higher levels of gear to be useful.  Demonology is good because it will improve you and your demons survivability, but it is low on damage for leveling.  It's for this reason I usually go with a demonology/affliction spec for leveling.  Affliction's main strength for leveling is the siphon life, which heals you each time Corruption deals damage.  This talent will keep you at 100% health all by itself usually, and is what allows you to life tap as much as you want (read: unlimited health and mana.)  Demonology's shining talent for leveling would probably be Soul Link, this transfers 20% of the damage you take, to your demon.  Having a well balanced spec that allows you to continually fight without stopping will help you level faster.  For a lock, smooth leveling is as easy as getting the talents I've mentioned.

I've been playing the Warlock class for the better part of 5 years and leveled my fair share of them. Of course things have changed now compaired to before thanks to talent tree overhauls and the implimentation of the LFD system.

If you're going to quest your way to 80, I recommend going Affliction. You can DoT numerous mobs, Fear/Howl of Terror them and /lol while they heal you as they take damage. Add that to Life Tap/Dark Pact and you have little to no downtime. If you're going to run dungeons, Demonology is the way to go. Affliction loses it's appeal in groups pre-70 because everything dies too fast for your DoTs to really be effective. Demonology lets you spam Shadowbolts for heavy burst damage and gives you numerous passive bonuses to crit and spellpower. Destruction is an awful choice for leveling since a lot of the bottom teir talents increase fire damage but you don't get your primary Destruction nuke (Incinerate) until you hit Outlands.