First of all, you're going to need a good party setup: warrior tank, healer mage, archer rogue are a must, since it seems you are playing as a damage dealer mage.
Your best bet for a tank is a sword & shield warrior, like Aveline. For her, you should focus on the Weapon and Shield and Defender talent trees, as well her unique talents in the Guardian tree. She's going to need every ability that draws enemy attention towards her and increases her defense. Immovable, Shield Defense, Stone Wall and Turn the blade are all great for defensive purposes. As for drawing enemy attention, you'll need taunt, and the aggressive or the tank behaviour in tactics.Set her tactics to rush in first, activate her defensive abilities and draw attention away from party members if they are being attacked. As for the skill points, you should increase strenght, dexterity and constitution. Give her the best shield and sword you find, and choose the trinkets which increase armor and defense.
For a healer mage - Anders in this case - you should focus on Arcane, Creation and Vengeance trees. Mind Blast from Arcane is useful for stunning enemies and reducing threat on your healer. Crushing prison is recommended, but not essential for Anders. From Creation, Healing is naturally essential, you should set the tactics to heal party members and himself if ther health drops below 50%. The other useful spell from creation is Heroic Aura, which is also useful in keeping your party alive. The glyphs are also useful. From Vengeance tree, you should evolve Panacea and the two spells that come with it, but usually you don't have to keep Panace active during regular fights, just prolonged or boss encounters, or when your party is really taking a beating, The Vengeance tree can be ignored, if you are building a healer. At level ups, spend points to magic and willpower.
For Varric, the Archer rogue, I'd recommend the following trees: Bianca, Marksman, Sabotage and Suberfuge/Scoundrel for threat reduction (Armistice or Evasion). The rogue is going to be a damage dealer, so you should spend your points to dexterity and cunning, From the Marksman tree, you should keep Bianca's Song active at all times, it gives Varric a good chance to hit his target and deal critical damage. From the Sabotage tree, you should have Miasmic flask by default, use it on clustered enemies to stun them and give your mages a chance to nuke them freely. From the Archery tree, go for everything and use them according to situation, the descriptions should tell you what the abilities do.
And as for your mage, with this party setup it should be a damage dealer & debilitator. You're going to need the Elemental, Primal, Spirit, Arcane and Entropy trees. First, you should use the abilites that weaken your opponents, and for doing this, the Entropy & Spirit trees are your best friends. Entropy spells are all useful, and you should also get Walking Bomb from the spirit tree, it can work wonders on groups of lesser enemies. From Primal, you should get Stonefist, Chain Lightning and Tempest; from Elemental, you can choose everything, those abilites usally the best damage dealers. From Arcane, Crushing Prison and Mind Blast are recommended. A generally good tactic for damage dealer mages is casting hexes and curses on enemies, then nuking them with powerful Primal and Elemental Spells.
Giving some basic healing spells to your mage makes Anders not so essential in every fight, so sometimes you can replace him with Fenris or Isabela to maximize your damage output. But never-ever remove the tank from your party, otherwise you are as good as dead on higher difficulties.
I think that's all. Following this guide and setting your party tactics accordingly should get you through the game. English is not my mother tongue, sorry for any grammar mistakes. Also, if something is not clear or you need more help, let me know.