How do I kill the trolls?

The main way to kill trolls is to hit them with fire spells as much as possible. Trolls regenerate health when attacked by swords or bows. Unless your destruction magic is high enough then just run.

Well there weakness is fire magic or anything that deals fire damage. Once hit with a fire spell then they will temporarily lose the ability to regenerate. You best bet, if your lower in level (25 and under), is to use a sneak attack with either a normal bow or an enchanted one that deals fire damage. Then switch over to dual wield two fire spells. Should make quick work of them.

Use a scroll of firestorm.... Works great

I just killed one with the flame spell, my horse, and a fist

Troll will regenerate health if attacked by anything that doesn't do fire damage. So have a full set of armor (don't matter if it's light, Medium, or Heavy), a shield, a weapons that is enchanted with fire and/or any fire spell. I killed a troll like this when I was level 2. So no your not weak but you probably have the difficulty to high for your experience.

To get rid of trolls you need to try to use a fire spell, this will stop them regenerating health. Get the perk that allows you to duel cast flames, then find a troll and duel cast flames whilst walking away from it, once you run out of power of the troll gets to close run away and turn around and replete the process.

Use fire. I recommend going to the college in winter hold to level up your magic so then you can use fire spells to kill them while backing away.