How to get started.
First of all you should read about Alchemy and its architecture in the guidelines.
The most important things to know about Alchemy are elements and page layouts.
Elements:
With Alchemy you can split pages into content parts, elements. These elements can be defined out of several base content types: essences. The basic essences are:

EssenceText - A single line of text
EssenceRichtext - A TinyMCE powered formatted text block
EssencePicture - A reference to an image
EssenceHtml - HTML embed code
EssenceSelect - A selection of values
EssenceBoolean - A checkbox

Elements get defined in a YAML file config/alchemy/elements.yml
Read more about elements and how to define them in the guidelines.
Page types:
You can define several types of pages, called page layouts. You can assign elements to page layouts and control how elements and the page of a certain layout behave.
Page layouts get defined in a YAML file config/alchemy/page_layouts.yml
Read more about defining page layouts in the guidelines.