DIY & Home Improvement

Easiest option is to put plywood on top of the joists (the wood I-beams). Unless they're spaced very far apart, they can easily handle the load. If it was a "normal" floor, you'd use inch-and-a-quarter plywood. If it's just going to be a storage area, you could use a little thinner plywood as long as the load wasn't extremely heavy.

If you want something that's under the joists, the easiest are various tray systems that are suspended from the joists. Some of them are fixed platforms, others are suspended by ropes so you can get stuff out more easily. If you google "garage overhead storage" you'll find plenty of options.