Okay, so you've just awoken THE BEAST because I absolutely love Dragon Age and will ramble about it for days on end. Sadly, I'm not 'the best' when it comes to combat tips but I can give you a few that haven't failed me yet. Can I ask what you're playing this on? I don't really think it matters but I've played the game on the PS3 so there may be some differences.
So I wrote a huuuge piece of writing for this but my laptop decided it was a great idea to delete all of this. Which is super bloody annoying, so I'm going to make this short and sweet. You don't have to follow all of this advice, because it's very much your experience and Dragon Age is great fun when you do things your own way.
Always have a mage in your party if you aren't one. Mages in Origins are fantastic because they have access to all the same skills, unlike in the rest of the Dragon Age series. Seriously they are so good, please take a mage with you at all times. They have crowd control spells that stop enemies from overpowering you like Mind Blast and Cone of Cold, and they have healing spells too.
Did you say something about always having injuries? Invest in injury kits for your party. Normally Bodahn sells them, but pretty much any merchant can sell you them too.
Alistair can be a meat shield if done right. I usually just put everything into con and strength for him. Give him second best armor (because I DESERVE the best armor) and give him skills that allow him to be offensive and defensive when the time calls for it.
I'll be honest, I've still not figured out Leliana yet even though I've been playing since 2008 lol. Dex and cunning for her can work well, especially if she's an archer, but her skill set as a bard is key too. She can really boost the strength/defence of your party. Look into her skills more.
Potions/poisons are so beneficial and really change the battle if done right. Invest in them if you can.
Sorry I can't help much more. If you're struggling with it more though let me know and I'd be happy to help!