As a couple of posters have said: As you're starting out - heal. Dps can be introduced when you get more familiar with the class.

But I'll also add dps is only secondary. If you're unsure about the group and there's been a couple of sticky moments - then forget it.
The role is to keep the group alive in the order of their roles - Tank then anyone else you can manage. That's it.

When your group has successfully finished, others won't care less about your dps ranking.
1. Heal first
2. Dps only if you are comfortable with 1
3. If anyone moans let them heal themselves.