How To Improve Your Soil.
You can't really change your soil's texture but you can improve it. However, it takes a large amount of effort and a long time to see any kind of permanent results. If your soil is extremely light (silty or sandy) you will probably want to mix in some fine-particle ingredients to make it less porous and more moisture-retentive. If your soil is very heavy (clay-like) you will want to loosen it, again making it more porous and better drained.
The ideal is, of course, somewhere between extreme lightness or heaviness. Such soils are called loams. Loam is a rather indefinite description as you can see, but implies enough large or coarse particles (like sand) so that porosity and drainage are good, and yet sufficient small particles (like clay or organic materials) to hold fertility, nutrients and moisture abundantly. Such a soil is almost automatically a good home for beneficial microorganisms.
One of the best ways to improve either a heavy or a light soil is through the addition of organic materials. This seems to be a contradiction, but as you'll see, it's easy to understand why it works so well.