Runlevel of choice. GRUB, ubuntu

I'm quite new to linux, and have a few questions regarding my assignment. 1. I must give a choice to the user in GRUB, to use either 3rd or 4th runlevel. May i ask for any materials and/or tips on that? 2. I have to make a certain service start automatically only at the 4th runlevel. In notes from my lecturer he mentioned putting rc scripts in /etc/rc.d directory, but i don't have one, with such name. Instead i have files like: rc0.d, rc1.d ... rc6.d. Are those the directories i should be putting my scripts in, and are these numbers in their names, corresponding to the runlevels? And also, i have an SSH, on my ubuntu on my VirtualBox. I want to be able to connect there through my host machine with windows, and i can do that easily, as long as PasswordAuthentication is on, cause when i turn it off and rely solely on public key, i get access denied. So the problem here is me not knowing how to apply the publickey to my host machine (ssh-copy-id doesn't work, tried also just echoing the publickey to authorized_keys in the proper localization, but it didn't work either) Any links with the information that i need, is also appreciated.