How do you write music? Are you sequencing with synthesizers in a DAW, or are you recording yourself playing on physical instruments. If you actually mean a metronome, then most if not all DAWs should have one built in, so that you can keep time while recording. But if you mean a drum track, then you could sequence a beat with drum samples.
Sorry, let me ask again. Do you record instruments or use MIDI?
Most UTAU users tend to use midi, because it's cheaper to use a computer than buy or rent instruments. However, I don't want to give advice that won't be useful if that isn't your case.