If you don't feel comfortable DPSing, then don't. As others have said, your primary job is to keep the party upright. Although, as you do dungeons, you'll become a lot more comfortable with how pulls typically go, your own skill, etc. Eventually you'll get a feel for things and know when to dps and when not to. As with anything in life, it takes experience. Just don't let jerks in DF discourage you.