To do this tutorial you need to know the note names.
Two notes are a semitone apart if there are no other notes between them (e.g. C and C#, or E and F).
To be able to spot semitones quickly, it’s useful to imagine
boundary lines on the piano: between B and C, and between E and F – in other words, wherever there’s a black note
missing. A semitone is then always a black note and a white note next to each other (e.g. C-C#), except when it crosses a
boundary line, when it’s two white notes (e.g. E-F).
(Note, if you’re revising this topic and are used to using the term
half-step rather than
semitone, you might want to read my post on why I choose to use the British variant of this particular musical term.)
Play lots of semitones on the piano, including ones that start on black notes and ones that are only made up of white notes.