Nice answer, Raka!
Laura, one of the first questions that came to my mind upon reading your question is whether your daughter expressed a desire to play soccer or whether you unilaterally decided she would love to do so.
Your daughter is a child, but that doesn't mean that she lacks intelligence nor that she is without feelings.
Your job as a parent is to teach her how to think and to make decisions, so she will gradually learn to become independent.
This is all the more important if she has a personality that is slow to warm up to crowds or new situations. How will she ever learn if she cannot experience?
You must start small. As Raka said, find out what SHE is interested in or would like to do.
Once you have identified that, contact the instructor or agency responsible for the activity and ask to be put in touch with the parents of one or two other children who will be participating in the activity. Finding a child of similar temperament to join forces with your daughter will do wonders to help her through the initial phase of the activity.
Again, I caution you not to jump to conclusions as to what activity will be "fun" for your daughter and that you think she will "love."
She may be only six years old, but she is her own separate person; with her own thoughts, likes, dislikes and feelings. Encourage her to find her own path and support her on the journey.
Meanwhile, I am not saying you're doing this but I must caution you (just in case):
Please do not try to live YOUR life through HER.
P.S. Once your daughter selects an activity she wants to try and you take her to its first meeting - don't stick around to watch! Take her there, make sure she meets the adult in charge and then LEAVE!
Don't make a big deal of it, just say good-bye and let her know you'll be back at whatever time you've prearranged. (You can always give her a watch so she can keep track of when you'll return.)