Do you have enough head room?
Check zoning and codes for your area, first. Headroom, exits, etc. are safety issues. You may have to make some structural changes.
What's the goal? Is the space insulated? What do you have to start with? Also check local building code. You may need to hire a contractor to make it a legal space addition. See if these help you.
Check the building code where you live. You'll have to make sure the trusses would support a floor, and you'd also have to insulate it. I'd get a quote from a contractor
We insulated and hung drywall, then put down flooring. We didn't change the pitch of the roof, just went with it.