To add to Lisa's suggestions,
I always suggest you use LESS CYA than the package says to use. CYA is easy to add, hard to remove.

I personally say use 1/4 to 1/2 the recommended amount--if you have 0 and know you have 0, you can use 1/2 the amount. After a week, you'll test for CYA and the see where it reached. I find the containers of pool chemicals are wildly inaccurate about amounts--particularly containers of Di-Chlor, Dry Acid, ....and CYA.

You have an 18' round. That means about 7500 gallons. I would add 2 quarts of regular, plain unscented bleach to start. That should bring you chlorine level to 3.-5 to 4 ppm (parts per million)--a good, safe level for no CYA. Lisa's right, though, that without CYA, the chlorine level may drop. That's why you need to test it everyday. Test it in the evening, so that the chlorine won't be broken down by the sun's UV rays. Add more bleach if the Chlorine level has dropped below 1ppm. 1 quart of regular bleach should do it--it will add about 1.75ppm of chlorine to your pool. 6% Ultra bleach will add a little more per quart.

This is just until you get going and can get us good test numbers.