In a plain positive sentence, some usually feels right.
some in a positive sentence
I have some books.
There is some water in the glass.
She bought some apples.
Not "I have any books." — say "I have some books.". A plain "I have" sentence is positive, so it takes some, not any.
Try it: She bought ___ apples at the market. (some / any) Answer: some — This is a plain positive sentence (she did buy them), so it takes some: some apples. any is for negatives and most questions (more on that soon).
any in a no or a question
When the sentence is a no (don't, isn't), or a question just checking if something exists, it usually turns to any.
I don't have any money.
There isn't any milk.
Do you have any questions?
Not "I don't have some money." — say "I don't have any money.". "Don't have" makes it a no, so some turns to any.
Try it: There isn't ___ coffee left. (any / some) Answer: any — "Isn't" makes this a no, so it takes any: isn't any coffee. Some is for plain positive sentences.
Try it: Do you have ___ brothers or sisters? (any / some) Answer: any — This is a question asking if they exist, so it takes any: any brothers or sisters. (A plain question turns some to any.)
Offers and requests use some
When you offer something (Would you like...?) or ask for it (Can I have...?), use some. These questions expect the answer yes, so they are not the "is there any?" kind.
Would you like some tea?
Can I have some water, please?
Could you give me some help?
Not "Would you like any tea?" — say "Would you like some tea?". You are offering, and you expect a yes, so it takes some, not any.
Try it: You are offering a guest a drink: Would you like ___ juice? (some / any) Answer: some — You are offering, not just checking if it exists, so it takes some: some juice. A warm offer uses some, even in a question.
Try it: Positive, or a no or question? We don't need ___ help, thanks. (any / some) Answer: any — "Don't need" makes it a no, so it takes any: don't need any help. (Plain positive → some; a no or question → any.)
Try it: The word can change inside one talk. Sofia offers: "Would you like ___ tea?" Then she looks in the cupboard: "Oh no, there isn't ___ tea left!" (some … any / any … some / some … some) Answer: some … any — The first one is an offer, and an offer takes some: Would you like some tea? Then "isn't" makes the second one a no, so it turns to any: there isn't any tea. Same talk, same word — the sentence around it decides.