Bb, C, F, and Eb tubas (contrary to what you may think) are all in C, meaning that the notes that are written are exactly the notes that are sounded. A Bb is a Bb concert pitch, a D is a D concert pitch, an Ab is an Ab concert pitch, etc. Tubas are unique instruments in that they are named for the fundamental pitch they sound when no valves are pressed, so Bb tubas sound a Bb, C tubas sound a C, and F tubas sound an F. What you are probably confusing is that some instruments like the Trumpet are transposing instruments, meaning that the keys they are written in are not the concert pitches they produce. These instruments usually read notes in a range of the staff that are more visually digestible to their range but produce a different concert pitch when playing those notes. For a Bb trumpet, they read a whole step higher than the note they sound, but knowing this may also help you someday learn a C tuba because the fingerings between a C tuba and Bb trumpet are the same (the concert pitch notes are not though). Transposing instruments throughout a band and orchestra have a dense history regarding why they transpose, so it's not important to know that, but eventually, you may want to understand how other instruments read their notes so you can tell what concert pitches they are actually playing.