I usually test again after about 2 hours and do another pwc (or not..) accordingly.
If you bought the decorations in a LFS and they are meant to be in a tank, they should be ok. If not, maybe someone else knows the answer to that...
Don't worry about cleaning the tank just now, get it stable first. Also, don't change the filter at all till you have the tank stable. I personally never change the filter, I just rinse it in old tankwater (when I do pwc) but lots of people change the carbon once every 2 to 4 weeks. Carbon will help your water to look crystal clean, I personally like the greenish tint my water has... You need the filter pad (biowheel, foam, whatever your filter has) because it houses a lot of healthy bacteria!
In all my fishy cycles, the amonia didn't take too long to go down (maybe three days) but the nitrites usually take about a week (with me doing twice a day water changes...).
Remember, you only have to learn this once!
L.