Click on the 'Documentation' link near the top of the page.
(1) Read through the tutorials
(2) Watch some short videos
(3) Glance over the Greenfoot APIs in the 'Reference' section
(4) At least, attempt to build a scenario
(5) Then, if you are having specific difficulties, start a discussion.
Best wishes and Happy New Year!
Danpost has outlined a good way to get started - of course the book is also around if you want to get / read that.
Other than that, we'll need to know what specifically you need help with if we're going to be able to give meaningful advice! Otherwise we don't know if it's a programming question, an interface question, what part specifically you've stuck on, what you've tried, what you're trying to achieve... it's akin to saying, "I can't use my computer, help!"