Building a shed
As a former lumber estimator for all sorts of commercial and multi-family projects, I'll tell you that most of the software out there won't actually get your project built in the real world. Buy off of a computer generated list and you're guaranteed that you will be back at the lumber yard buying more for miscellaneous use, even if you're not wasting much of anything. It isn't all that easy if you don't have a deep understanding of how everything goes together.
To be safe, figure up every stick and sheet of material you think you'll need and then add 15% or so. Also buy enough 2x4 for temporary bracing of the walls. Count on things like 4 to 6 studs per opening, and 3 studs at corners. Buy pre-cut studs (not 8 footers) and also get plenty of extra random length plate stock to do a double top plate and frames around windows and such. If you're using trusses, be sure to get plenty of extra 2x4 material for bracing. Little things like subfascia, frieze boards, drip edge, anchor bolts, sillseal, etc. will add up to a lot.
My suggestion is to break your building up into sections. Start with slab materials. Then go to walls and openings. Then to the ceiling/roof framing.
Finish estimating one phase at a time before moving on to the next.
It helps to get a book that shows how a building is framed, even if you know what you are doing. I use the book "Architectural Graphic Standards." Even if you know what you are doing, it helps to have a diagram of a framed building in front of you so you don't accidentally omit something.