You can tap along to the beat using this website:
https://www.all8.com/tools/bpm.htm
I recommend tapping on the table for a few seconds to get the beat of the song before you start tapping on your keyboard. This will help make sure the rhythm is already stable.
Given that the song you've chosen is well known, you're likely to already find a premade UST file, a MIDI, or a file for other vocalsynths like Vocaloid. If you need to convert the file, you can use UtaFormatix:
https://sdercolin.github.io/utaformatix3/