Thanks guys for your good advices.
About me singing the first hour, please let me explain some more.
I agree with you that I do not want to inconvenience the audience. You're absolutely right, they are first, I'm second.
My idea is to let the people know that the Karaoke session starts at for example, 8 PM, and it will last for 3 hours.
What I will do is come a little earlier, sing for an hour, starting at 7 PM.
Why?
The place is a latin restaurant. The one-hour-me-singing-session is to have someone "live" singing mild latin music, that is appropriate for dinner time, instead of hearing to a CD or radio for a change.
That's all.
The Karaoke session will start an hour later, at 8 PM, and last for 3 hours.
BUT, if during my singing session, someone wants to sing a song, I will let them, go ahead, no problem. And the Karaoke session could start from right then.
Gracias de nuevo for your comments.