So, I'm currently running a Human Ranger, and a Human Barbarian. For my next game, I'd like to get into the world of magic use.

Now, of course, I don't know which version the next game will be, Pathfinder, 4e (maybe 3e or 3.5, but more likely PF or 4e).

Any suggestions for a first time magic user, as far as class/race/skills/feats go? As stated, it will be my first foray into this, so, I'm really just wondering what class would be best for a newish player, and what races I should look at? I see alot of people playing Tieflings - is there a specific reason that race is so popular?

I might recommend something like Cleric. You will still default to beating people in the head a lot of the time, but you also get the flexibility or rearranging your spells each day, so if you find out something sucks, you aren't stuck with it until you can retrain it, like a Sorcerer or Wizard. Pathfinder, obviously.

As for Tiefling? I don't know. Some people might like it because it can have several different racial stat modifiers in Pathfinder, others might like the edgy look and personality. I like them because they have immense versatility in Pathfinder, I can toy with size, racial attributes and appearance nearly as much as I want.

As a first time magic user, I wouldn't play a Cleric or Druid right away. Anyone with a divine list is just gonna seem overwhelming. For comfort, a Magus could be nice. It's a hybrid of Fighter / Wizard and it allows you to swing a sword or cast a spell, sometimes at the same time.

~SE++

Well, definitely not Druid, because you have to keep in mind statblocks for Summon Nature's Ally and Wild Shape. I don't think Cleric is all that bad though.
Can't give an opinion on Magus. Never bothered to read all their class features.

You've already played Ranger, so you should have the basics down. It's not that big of a learning curve, and you can always ask your party what they think will be useful.

I will say, the people I've been playing with here have been super helpful. We've got a player who prefers clerics...Druid does seem complicated...I do like poking things with long sticks.
Where's paladin on the complicated list?

I've never played a caster role either. Ive been wanting to do one via play by post since its slower and allows for more rule questions.

I wonder if theres a "casters for noobs" guide somewhere that assumes you know nadda.

Paladin's aren't casters in the traditional sense. While they do get a limited amount of spells to throw around they have to be fairly high level and are mostly restricted to personal buffs and the like.

Now this is going to be entirely subjective but as far as I'm concerned the complexity of spellcasters for new comers looks something like this*;

Cleric's are a good choice for beginners. While you wont be throwing fireballs around your be a solid member of any team by providing buffs, heals and general good will for the others. Now this isn't to say a cleric is entirely passive; any cleric worth his salt keeps a few summoning spells on him to call down divine aid in times of trouble and at later levels you can cut out the middle-man and just rain divine justice down on your enemies. Of course the biggest advantage of a Cleric is once your spells are spent your still good to contribute to a battle. Your not as well trained as a fighter but you are still heavily armoured, decently armed and more than capable of holding the line when the chips are down (which is more then some other spellcasters can say).

Bard ain't full spellcasters in the same way a Wizard or Cleric is but they still get enough spells under their belts that I'd class them all the same. The Bard's main advantage is the variety of spells he can muster; heals, buffs, illusions and enchantments all fall under his skill set. In addition the Bard is a great stepping stone from a fighter or skill-based class to full spellcaster as they are masters of versatility - you may not be front-line but you can still contribute to a fight as well as having the second largest skill pool in the game (albeit focused more of knowledge and social skills). A good class to get to grips with spellcasting as a system. The only real issue is that Bard's are spontaneous spellcasters so you have to pick your spells when you level-up rather than just learning everything but I'm sure there will be those of us here willing to talk your ears off about your spell choices.

Druids shares many of the same bonuses as Clerics; there reasonably tough once there spells are gone, have a good number of skills that are useful out of combat and get a huge number of interesting a fun special abilities (animal companions and shapeshifting spring immediately to mind). In terms of spellcasting they also have a surprising amount of variety in there lists. You can heal and buff almost as well as a cleric while still having enough blasts to go on the offensive and some really fun utility spells that reward clever use.

Wizards are a prime example of the old saying 'good things come to those who wait'. At low-level there really nothing to write home about - low hit-points, as tough as a sheet of paper and only a few spells per day makes them usefulness-impaired if you don't know what your doing with them (pro-tip: a 1st level Wizard who prepares 3 copies of Magic Missile is doing it wrong). At higher levels however a skilled Wizard can shut down entire encounters with a few well-placed spells. A Wizards true strength is in his spellcasting versatility - given time and resources a Wizard can learn almost every spell in existence and with some forethought can be prepared for just about any situation. Overall risky to start out with as mistakes will be made (and can be costly) but they are (in my opinion) one of the most rewarding classes to control.

Sorcerers are a odd option for new comers. They combine the Wizards frailty with a Bards relatively limited spell options. This isn't to say they can't be good (even brilliant) but you tend to need to know exactly what your overall goal is when building a Sorcerer as your spell options are with you for your adventuring life and you don't get the Cleric/Druid/Wizard's luxury of just swapping around your spells each time you rest. Not a bad class or anything just the highest on the 'need to know what your doing' scale.

*FYI this is just covering the base classes as they appear in both 3.5 and Pathfinder - if your interested in Pathfinder specific spellcasters just ask and I'll throw up some info for 'em.

4e casters play basically the same as any other class, deoend on which role youre trying to fill. For the most part though they will move and attack similar to your ranger. Once you move into D&D 3.5 or PF things get way more complicated. Spells per day, pray to gods for spells vs meditation, etc.