Any Chrome extension starts as an unpacked extension: a folder containing the extension's files.
One file it must contain is manifest.json, which describes the basic properties of the extension. Many of the properties in that file are optional, but here is an absolute minimum manifest.json file:
{
"manifest_version": 2,
"name": "My Extension",
"version": "1.0"
}
Create a folder (for example, myExtension) somewhere, add manifest.json as listed above to it.
Then, you need to load the extension in Chrome.
chrome://extensions/ page, accessible though Menu > More tools > Extensions.myExtension folder.

That's it! Your first extension is loaded by Chrome:
Of course, it doesn't do anything yet, so it's a good moment to read an overview of extension architecture to start adding parts you need.
Important: When you do any changes to your extension, do not forget to return to chrome://extensions/ and press the Reload link for your extension after you make changes. In case of content scripts, reload the target page as well.