There are different text styles which you use for creating types of text.
EG: expression, lyrics, chords, tempo, figured bass, etc.  (Dynamics are
created using the expression text style.)  You can find all these in the
create menu under text.  As you browse the menu, take note of the shortcut
keys.  The most useful are Control + E for expression, Control + T for
technique, Control + L for lyrics and Control + Alt + T for tempo.

Don't confuse these text styles with fonts.  Text styles are far more
advanced than that, although each text style does have a font associated
with it.  In other words, each text style will appear in the font which it
has been assigned.  Most text styles will use a combination of times new
roman and the opus font which is used for musical symbols.

Text items can be system items or staff items according to type.  Tempo,
titles and rehearsal marks are all system items because they appear in all
staves, whereas expression, technique, lyrics and chord symbols are all
staff items because they only appear above or below one staff.

To add a text item you must first select the note which is closest to where
you want to text to be created.  If you're creating system text then it is
enough to choose any note which has the same rhythmic position that you want
the text to have.  For staff items you must select the note in the staff
that you want to text to appear in.

Once you've selected the closest note, use the create menu or press the
shortcut key for the text style.  The text item will be created and an edit
window becomes active which you use to input the text.

Once you've finished entering the text press Escape.  The edit window closes
and you'll be placed back in the score with the text item selected.

If you want to edit an existing text item you'll need to select it first and
then press enter to activate the edit window.  System text items can be
selected using the system items dialog (Alt+F3) whereas staff text can be
reached by pressing Tab or Shift + Tab when you've selected the note which
is closest to the text item you need.

Sibelius will take note of any text you input and adjust the playback of the
score accordingly.  For this to work you must make sure that you spell words
properly.  For this reason there's a pop-up menu of common terms which you
can open by pressing the application key, Shift + F10 or the right mouse
button.

The menu of common terms will be different for each of the different text
styles.  The menu is actually a property of the text style and you can edit
it to add your own terms if you really feel the need.  There are also
shortcut keys which you can use within each text style to add a term from
the menu of common terms.

One thing to take note of is that some of the common terms use different
fonts with specific sizes and attributes.  For example, "p", "f" and "rfz"
all use the opus font.  This font looks like a rather elaborate version of
times new roman but it is larger, bold and italicised.  On the other hand,
"dolce", "legato" and "marcato" all use the standard times new roman font in
italics.

As a brief example, here's how to add "ff" below a note.

1. Select the note.
2. Press Control + E to create an expression text item.
3. Press Control + F twice to add two "f"s in the opus font.
4. Press Escape to close the edit field and return to the score.
5. Press Tab until you have selected the next note in the staff.

Tempo text is left-justified above the note or rest
Repeat text is right-justified over the note or rest.