The other side of fear ...
An old saying is that the other side of fear is desire. It's the fear and desire felt at the same time that makes it exciting. I think it will help if your partner is an expert at tickling you first, and this may take time and practice, which is fun anyway.
I was scared to be tied and tickled the first time. In fact, I had some lady friends who were in the same boat as me: we had lovers who tickled us mercilessly every day, and they wanted to tie us and do it. So we shared our feelings on our jitters about it. What was the big deal, we said, we're getting held down and tickled endlessly anyway? The difference is submitting voluntarily. That's really asking for it.
Eventually, our desire overcame our fear, and eventually we all let ourselves be tied and tickled, and we shared our experiences with each other. We liked it so much, we wanted it again and again. The fear was sort of still there, but it turned into anticipation, and tied up, there's a lot of anticipation. I think half the fun of tickling is the anticipation. Sometimes we'll schedule the tie and tickle as much as two weeks in advance. That way we go nuts with anticipation. Of course, there's spontaneous tie and tickle. That's fun too!