“Where can I” is the correct structure when you are asking a direct question. When asking a direct question you always change the order of the subject and the main verb.
“…..where I can” is also correct if the question is an indirect question. An indirect question has some additional words before “where I can” and is a way to be more polite in English.
Where can I find the nearest supermarket?
Would it be possible for you to tell me where I can find the nearest supermarket?
Notice that both examples have the same intention but the second example adds extra information. This
Where Can I?
“Where can I?” is correct when it comes at the beginning of the sentence.
Where can I find some meat for my taco?
Where can I go to the bathroom?
Where can I stay tonight?
Where can I buy some beer?
Where can I find a nice girl?
Where can I work now?
Where can I dance the Tango?
The above examples are in the normal structure of interrogative sentences
Where I can?
“Where I can” is correct when it forms part of an indirect sentence.
Do you know where I can buy some tennis shoes?
Can you tell me where I can find a nice girl?
Do you mind telling me where I can find the bathroom?
Could you tell me where can I fix my phone
Would it be possible to know where can I see a doctor?
If a question is open or closed, it is usually a direct question and an interrogative sentence(you use a question mark). An indirect question is when a question is part of a larger sentence. You need to use a question mark only when the sentence starts with a question asking word(do, can, would, etc)