If you don’t know if you need to use there is or there are at the beginning of the sentence, you need to look at the following noun – if it’s a singular (a cat for example) you use there is, and if the noun is plural (two cats) you use there are.
Generally speaking, some is used in positive sentences and any is used in negative sentences. Any is also used in most questions, but some can also be used in questions when we think the answer is positive (for example Would you like some chocolate?)