Thanks for writing. If you have hard, caked soil there are two routes you can go with creating a garden. The first you alluded to --- build a raised bed. That's fairly straightforward. Just use materials (bricks, stones, wood timbers, etc.) to outline your bed and make a frame. Then fill the frame with good-quality soil. You can make your raised beds as deep as you want to --- even as little as 6 inches above the ground can make a huge difference in how well your plants grow if you fill the bed with good soil.
The other route would be to amend lots of organic matter such as compost, coir, or peat moss into the soil. This will help break up the soil and keep it from drying hard after it gets wet again.
Either way, you can plant as soon as you're done mixing in organic matter to the soil. There's no real wait time.
---Justin, Senior Garden Editor, BHG.com "