Well first of all, welcome to OFPEC!
Secondly, If you intend to only use text to communicate your lines, it's as simple as using the
CutText,
TitleText,
Sidechat, or
Hint commands to print the text to screen.
If not, your task becomes much more involved and you are at the beginning of a wonderful journey.
The Description.ext is a file that goes in your mission folder to define certain attributes of your mission like sounds, gear selection, music, character identity, and other resources like dialogue, etc. It also defines mission flow for campaigns. Learn more about it
here.
You'll need access to audio recording software by which to record the actual sounds.
And to make the mouth of the models move in unison with the sound files, you'll need to use
wav2lip by BIS
Once you have the files in the sound folder, and they are defined in the Description.ext, you'll have to use commands to call them.
SAYPlaySoundPlayMusicGood Luck