To decide which type of text you need, visit Text types.
For general-purpose "text boxes" attached to staves, use Staff and system text, they are good if you simply want a text for visual purpose.
To add a text-based element to the score, use one of the following general methods:
Notes: The exact method depends on the type of text you are adding, visit and follow links on Text.
To know more about property fields in the Inpector and their effects, visit Text styles and properties
When a text object is created in MuseScore, it assumes the corresponding Text Style: all staff text, for example, has a unique Text Style, as have tempo markings, dynamics, chord symbols etc. Each Text Style consists of a number of text properties (font-face, font-size, font-style etc.), which can be viewed, mainly, in the Text Styles dialog (Format→Style…→Text Styles).
A newly-created text object starts off with identical text properties to the Text Style. These can be viewed in the Inspector:
However, these text properties are not limited to those of the default Text Style: if need be, you can edit them in the Inspector independently of Text Style.
Finally, if you want to apply formatting to specific characters in a text object (font-face, font-size, Bold, Italic, Underline, superscript, subscript), you can do so in text edit mode.
To position a text object, use any of the following methods:
When you apply a text element to the score, its anchor position will depend on the type: