In case you don't know what the Build button is, it's this one.
Lower left of your screen. Then look for the image in shMike's post at top center of your screen. Click the Move button and then click the building/decoration you want to move. The building/decoration will then follow your cursor. While you move the building/decoration you will see a red/green shadow on the city area. When the shadow is green, it means you can place the building/decoration there. Experiment and you'll quickly get the hang of it.
To answer your latter question, in case you ever decide to remove a building/decoration, click the Build button, then click the Sell button indicated in shMike's image, then click the building/decoration you want to sell. (Most of the time) you'll receive some resources back. Warning: the building/decoration is then gone forever.