Can I lay down plywood in my attic to create a storage space?
As far as the opening goes, you can put in a collapsing ladder, such as this one. I've installed one before and they're not too bad, just a little framing work and some large nails (I used 16p nails and you'll need a friend or two to help, the ladder and frame is somewhat weighty). This gives you a ladder and an easy way to get up to the attic and it collapses down to just a panel which can be painted and framed to make it look nice against the ceiling.

The attic in my house just has some plywood on the truss floor which serves as a great storage floor. Just have to be sure not to block the air flow in the attic and realize that it may get very hot in the summer and cold in the winter. I made the mistake of storing some candles in boxes one summer ..... wound up with candle blobs when I discovered them again.