It is very important to be conscientious of others if you happen to go with Japanese friends. Although nobody will directly tell you this rule, people usually take turns singing at karaoke. No one will probably say anything but you may get some cold looks from your friends if you dominate the mike and thats not how to enjoy karaoke. Also, if you are with a group of friends, it is mannerly to avoid singing songs that no one knows. Singing a song that everyone can sing along with is ths best since everyone gets excited. You can also easily change the key of any song to fit your vocal range and so you don't have to strain too much to hit notes that are too high.
Karaoke is fun. You HAVE to try it during your visit to Japan!