"Good instructor" is such a broad term. Every instructor has strong and weak areas, especially starting out, but that's life.
Are you referring to not knowing if you'll be able to explain things adequately, or not being able to communicate your ideas well enough? In that case, no, I wasn't really nervous. I've always had the gift of gab and have been told I have a nack for explaining things, so I wasn't worried about that part.
What I was worried about was the responsibility that comes with making sure a student knows everything they need to know, both legally and practically speaking. Aviation is so vast it's mind-boggling. There are so many things that can come up that could cause problems for a student when flying solo. All the little details you haven't thought about in your own training stand out when you have to teach somebody else. Handling emergencies, dealing with ATC, analyzing weather, navigating on XCs, the list could go on and on. It's a challenge to prepare somebody to be a pilot in a reasonable amount of time. There is a fine line between pushing them out the door before they're ready and doing excessive training, wasting their time and money.
As long as you care about what you're doing, work hard, and don't be afraid to ask other CFIs for help, you'll do fine. The learning curve was pretty steep for my first 100 hours of dual given, but then I got fairly comfortable with everything.