Before we get started I’d like to point out that a Professional Engineer’s standard of care typically includes a site visit to assess field conditions and get an overall understanding of the structure. This can obviously not be accomplished through the internet. The information provided here is meant for planning purposes only (general sizing and budgeting) and is based on the information provided by you. All loading cases considered are for vertical loads only, no lateral analysis has been completed. The information should be verified by a professional engineer who can visit the site to ensure that potentially important information has not been overlooked or omitted.
I can give you a couple of options:
1. If you construct your concrete filled posts at the corners of your 24'x24' storage shed and then in the middle of each wall, that will give you 12' o.c. spacing. I would then span the 12' in-between each post with beam made of Triple 2x10 Pine Plank beams. Your 24' roof trusses can rest on top of these beams.
2. If you construct your concrete filled posts at the corners of your 24'x24' storage shed and then 2 more in each wall, that will give you 8' o.c. spacing. I would then span the 8' in-between each post with beam made of Double 2x10 Pine Plank beams. Your 24' roof trusses can rest on top of these beams.
I have assumed a Roof Dead Load of 10 psf and a Roof Live Load of 20 psf. If you live in an area with snow load, then let me know and I can redo my numbers.
You can run your wall planks either vertically or horizontally, since they will not carry any load, due to the fact the your post and header beam system will carry your roof loading.
You will need to have a local engineer detail your connections, and determine foundation requirements.
If you feel you have received a satisfactory answer to your question, click the Rating button that is appropriate. Experts are credited for each adequately Rated answer they provide. If you have additional questions, please let me know. Thanks