Naomi's answer is correct. Second one: ～ではありません。is a normal polite version. ～じゃない。is a casual version. This is not my watch. これは、私の時計ではありません。／これは私の時計じゃない。same meaning.

Hi Mistie! I'm learning Japanese too, but I just recently learned this section from my textbook, so I think I can help out with the first part of your question. これ means "this one" and we use it to talk about things we don't name. For example, これはいくらですか。-- How much is this? この means "this" and is used with a noun. For example, このとけいはいくらですか。-- How much is this watch?