Difference between 大好きです and がすきです
Hello, can someone help me and explain me the difference between these sentences which both mean
"I like". And I have a question concerning the negation

Affirmative:
くだもの　がすきです
くだもの　だいすきです

Negative
くだもの　は　すきじやないです
くだもの　あまりすきじやないです。
// Why do I use ありま in the second. I am a bit confused

Question

Difference between 大好きです and がすきです
Hello, can someone help me and explain me the difference between these sentences which both mean 
"I like". And I have a question concerning the negation

Affirmative: 
くだもの　がすきです
くだもの　だいすきです

Negative
くだもの　は　すきじやないです
くだもの　あまりすきじやないです。
// Why do I use ありま in the second. I am a bit confused

Answer

Affirmative: 
くだもの　がすきです　I like fruits.
くだもの　だいすきです I like fruits very much.

Negative
くだもの　は　すきじやないです I don't like fruits.
くだもの　あまりすきじやないです。I don't like fruits but I don't hate fruits.

あまり〜じゃない It's use for Negative sentences.　あまりmean "a little".

Answer

The differences in these sentences are both a matter of degree. Regarding your fruit appreciation sentences, the first sentence says you like fruit, while the second with 大好きis YOU REALLY LIKE FRUIT!
Regarding your dislike of fruit, the first sentence says you do not like fruit, while the second sentence is tempered somewhat and might be translated as “don’t like fruit that much”.