UTAU has the handy ability to import a track from a MIDI file, which makes things easier if you do have a MIDI file from a song.
Firstly, be sure to look at the MIDI file's tracks. A regular MIDI file can contain anything from guitar riffs to piano solos. In order to view the file, additional software may be needed. An easy pick would be NoteWorthy Composer, the Evaluation version is fine as the only thing needed from the software is to look at the notes in the file. Once downloaded and installed, open up NoteWorthy Composer and after clicking through the Evaluation notification box and the Tip of the Day, click and drag the MIDI file into the program. Click through the screens, not changing anything, and the musical staffs will come up. Find the vocal track and take note of where it is: first row, second row, etc.
Once the vocal track has been located, open up UTAU. Click on the first menu (ファイル, File) and click on インポート, Import. Find the MIDI file and click on it. A box opens up with numbers. Remember what row the vocal track was in? Click on the corresponding number for the vocal track and click OK. The notes come up, but the lyrics are all "a"s. Each note needs to be edited for each syllable. That's it!