Can someone share their knowledge with me on how to calculate load capacity on joist etc?

My building has a loft. I have 2X6's for the joist, 2 feet on center, with 3/4" tongue in groove sheeting.

I have wondered just how much load I can safely put up there without overdoing it.

Since I am far from an engineer, I don't know where to look to try and figure that out.

Any help would be greatly appreciated.