How do authors create 'mood and atmosphere' in their writing? Any helpful websites?Thanks

Use a lot of descriptive adjectives.
Like in horror films you would want to use a lot of chilling suspense type adjectives.
In romance you would want descriptive fiery warm adjectives.
See if there is a creative writing course available in your area like maybe at a community college.

This is all I could find that might help

http://www.webenglishteacher.com/creative.html

Scene (fiction - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Scene_(fiction))

Awesome Library - English (http://www.awesomelibrary.org/Classroom/English/Reading_and_Writing/Creative_Writing.html)

Teacher Resource Exchange (http://tre.ngfl.gov.uk/server.php?request=cmVzb3VyY2UuZnVsbHZpZXc%3D&resourceId=9876&output_register=html)

You might also try finding a book on creative writing.
Also look up synonyms for words you want to use.
Like if you want to say the dark wood walls
Looking up the synonyms you might come up with something like
The gloomy rustic walls
Or the rich vibrant mahogany walls

Synonyms Thesaurus - Letter A Page #1 (http://www.synonyms.net/synonyms/A/1)

Authors use words and phrases to guide the reader. In a movie you see this through the background music, theme song, etc.

I found a few online lessons, hoping that this will assist you in your
Question -
lesson17 (http://volweb.utk.edu/school/bedford/harrisms/lesson17.htm)
SaskEd : CW 20 - Module 3 - Writing Fiction - Setting (http://www.saskschools.ca/curr_content/CrWriting20/module3/setting.html)
create atmosphere mood (http://www.exampleessays.com/essay_search/create_atmosphere_mood.html)

Megan's Books: Editor's Essay, Tips on Novel Writing (http://www.megansbooks.com/essay.html)
Writing Well: Color My World — Infoplease.com (http://www.infoplease.com/cig/writing-well/color-my-world.html)
The Art of Description: Eight Tips to Help You Bring Your Settings to Life (http://www.writing-world.com/fiction/description.shtml)

Some links for you.

How do you talk about the mood and atmosphere in a text?

You should use adverbs and adjectives but I could not find and adult websites for you so I got:
Www.bbc.co.uk/schools/ks2bitesize/
Type this in the search bar and it should help a little bit

You can use alliteration with the sounds that you find capable of explaining the mood.vocabulary and rich language is something that is very important to master and different litterary features.similes, metaphores, anomatophea, alliteration and dicsription of setting all help to set the mood. You should read some of Kate chopins short stories or some classic books recommended by literature teachers.
Not sure if this is good advise because I am still a student at the age of 13 but hope this helps!