There's some great ideas here:
Its great that you are so enthusiastic about doing agilty with your little dog. It's not just a sport for the larger, athletic breeds at all, in fact ANY breed can enjoy it as long as they have the drive to do it. It must keep her very it and of course, any work with your dog strengthens your bond.
It might be worth contacting that agility club about your concerns, you might just be surprised. They might have a beginners class that you could attend, and they might even cater for smaller breeds. I am assuming you mean the person who runs the club is German? It would be you taking your dog round the course.
Another good idea might be to try and find an agility group or forum on the internet, so you can speak to likeminded people and find friends who also have little agility dogs. You might also find others in your area who could point you in the right direction to a suitable club.
I tried a fun charity agility course with my Staff mix, Elliot, once and the results were hilarious! I ended up completely pooped!
By the way, Shih Tzu's are adorable, so would love to see a pic. Good Luck!