There's nothing to it, all you do is choose a ride, place it anywhere, make a line to in, make sure the park is open, and people will ride you ride!
yes
You can download it here: http://download.cnet.com/Roller-Coaster-Tycoon/3000-7551_4-10026882.html for a 40 minute free trial and then you can purchase it for $20.
i'm not sure about online but you can download a free trial on loads of websites
It is not possible to play RCT3 without installing it via a disk.