A Bb-tuba is a transposing instrument. Music for tuba will be written using a bass-clef and one whole note higher then sounding.
To convert treble clef music to be played with a tuba use two part modification tools
transposition from C to B-flat
clef-change from treble-clef to bass-clef
To create a basic tuba-part from a lead-sheet containing chord symbols start as follows:
select the automated bass part
set transposition right by means of the C-to-Bb modification tool
reduce rests if nessecary rest-reduction modification tool
edit the result to your liking
See also: how to arrange music