I was also wondering where to learn to read the notation used in missals- I have never seen music written that way.
I presume you're talking about the 4 line staff, instead of 5 lines in Western music?
The modern standard grew out of the older, 4 line staff. The little "c" figure at the
beginning of each staff is the "Do" or tonic note of the piece, and therefore indicates
the key signature, so to speak. But the 4-line staff does not have a set pitch for
each line, usually. That is, compared to Western standard notation, A, which is
the second space up from the bottom in the treble clef, has 440 beats per minute.
But in the Gregorian 4-line staff, no such definition is standard, so you can effectively
change the pitch of the lines to suit your needs, or the range of the voices who
sing the piece at hand.
If you want to learn to sing Chant, you really need to join a group that rehearses
regularly, and you should go and study with them. It takes a few months of weekly
lessons before you get the hang of how things are done. And it takes years of
study before you become proficient, unless, that is, you have a talent for it that
lets you pick it up quickly.
Also, it helps to listen to recordings. Most recordings of Gregorian Chant are done
pretty well. I find it odd that a lot of churches with choirs singing Chant do it rather
poorly, and some are quite unbearable. The different thing about Chant is, you can
be a very experienced musician and yet have no clue how to sing it. Chant takes
a concentrated effort to learn well, and you have to put aside a lot of the
standard preconceptions that modern music entails almost inherently.
If there is any one rule to learn for Chant that is different from most other music,
it is, that at the ends of all phrases, reduce volume, or "soft on the ends." Chant
sounds like something else if that is not done.
Second, there must be continuity, or "line" to the singing, which means you do not
get to take a lot of breaths, and when you do breathe, it should be very fast, so
that you imitate an organ that plays without ceasing. There are some small
pauses, but not many, and it is commonplace for singers of Chant to do it
improperly by adding too many breaks.
And third, the tone of voice sounds much better if it is not sharp or edgy, but
smooth and round, which allows all voices to blend with each other, so that no
single voice stands out.