I've been going on a Japanese course and I don't understand the grammar behind 'dewa'. I've seen it being used in 私はおちゃとこひがすきではありません and 私はにほんじんではありません and I don't completely understand its use in grammar and definition. Is it the negative desu?
You are correct that ではありません is the negative form of です. At this stage I think you should just understand it as a single unit rather than looking at では and ありません separately. But See this link.
You will also see じゃありません (じゃ is a contraction of では) and じゃない (ない is the informal form of ありません) with decreasing levels of formality.
Note that it can be used to negate nouns and na-adjectives (as in your two examples), but not i-adjectives and verbs.