1.) Don't waste money on the specialization manuels. Don't get me wrong. BUY THEM, THEN RESET YOUR GAME. The game keeps track if you you've ever learned them in any of your games, not if you currently have them. So instead of wasting very hard earned money, buy it to unlock it for every game, then just reset. You'll still have it unlocked. This also goes for (I'll try to minimize the spoilers here) a time in Redcliff were you can go into the Fade and are offered gifts, one of which unlocks Blood Mage for mages, and later in the same quest were you can do something bad to get someone else to teach you the Reaver spec. for warriors or something good to get the Champion spec a little bit later. Save before those moments (multiple saves at different time recommended, just in case), do the "evil" thing, get the goods, and then reset to get something else.
2.) If you want to be able to choose more conversation options (especially the "evil" ones), get the Coersion talent on your character maxed ASAP. It should really be up to lvl3 before you enter the elve's forest (forgot the name. Starts with a B, I think) if you want to be able to choose the "evil" path. If you just want to be a goody-two-shoes all the time, then this talent has far less (though still some) importance.
3.) You will be able to get pernament stat increases when you go to the Circle Tower on your main quest (not intro mage). The most important of these stat increases would probably be the four cunning boosts that you get because it is often enough to give you the max requirements for the 4th level coersion ability (16 cunning, I believe) without wasting level up stat increases on a stat that only rouges use. This may be a reason for you to go to the Circle Tower first before anywhere else. If you're a rouge, then it's just a nice bonus.
4.) LOCK PICKING IS USELESS!!!!! Really, it is. The only items that you get in chests are a bunch of trash items, maybe potions at best. This isn't like Baldur's Gate or Neverwinter Nights. Chests are pointless.
5.) Stealing is only useful for a small, irrelevant series of sidequests. Your choice if you really want it. I think that you only need to have one leave to unlock the quests, and then can use other rouge characters to actually carry out the stealing if you want to do them but not waste the points.
6.) Constitution is only ever really good on your tank (maybe...) or, where it really shines, on a mage with the Blood Mage specilization. When you activate the blood magic mode, you use health, not mana, to cast your spells (discounted at 30% I believe, though I've never tried it...yet).
7.) Unless you use a mod, YOU CANNOT RESPEC YOUR CHARACTER. Once you select "Yes" you're stuck like that. This isn't so much of a problem with warriors or rouges since their abilties "trees" pretty much carry everything that is useful for the play still (sword and shield, 2H, archery, duel wield, ect.), but this royally screws you over if you're a newb playing a mage. Since mages don't follow one easy build (rather, they choose from multiple trees), it is very easy to create... not a bad mage, but a rather blaise, only partially effective mage. It won't suck... but it wont be good either. In otherwords, either get the respec mod (my preference) or do a lot of research on your character build, especially for mages.
8.) SHAPESHIFTER IS UTTERLY WORTHLESS. Unless there's some really good mod out there for this, shapeshifter for mages is utterly worthless. This is for several reasons. First, once you shapeshift, you drop all of those party boosting sustainables that your mage has probably aquired and would be useful to have activated (ex. frost weapons). Secondly, your attacks are based on your strength. REALLY?! What genious thought that one up?! As a mage, you'd be a fool to ever increase you strength. Even Arcance Warriors have their junk based off of the magic skill. Long story short, don't get shapeshifter unless you've got some good mods.
9.) Your origin story actually plays a small part in the overall story (especially if your human, though my favorites are the dwarfs). It's nothing to base a game around, but it does add a bit of fun to everything. The stat differences between all races are so miniscule that it doesn't even matter. So if you're deciding between that human warrior, dwarf warrior, and elf warrior, just think about what background story you're interested in this time around.
That's all that I can think of for now. Good luck and have fun.