If you're new to songwriting, you might want to check out this resource post. It has a list and links to many options for music software, instrument plugins, effects plugins, and other useful tools.
Before you can begin, you'll need a piece of software to do all of your musical work in. Somewhere you can have multiple tracks of audio or sequenced MIDI notes, where you can apply effects and have everything musically in time. This is a Digital...
utaforum.net
In whatever DAW you're using, you should be able to add an additional MIDI track to your project that contains the melody you want for the vocals, then export it as a MIDI file to use in UTAU.