There are always different ways to formate a sentence, even though each of them means the same you need to learn when to use which one. For example, it would be " my sister told me that her secret was", we can also say "my sister told me about her secret which was" it's a bit lengthy but more adequate to say. Moving on to the second one, this one is in passive voice and it should be " a friend of mine told me where can I buy tickets". There were two mistakes, first is when your using mine that means you are describing that this person belongs to you or know you in some way so you don't have to choose my in the starting and the second mistake is if there's a question between your sentence then you use verb first (can) before the subject( I ). Also the active voice of this sentence would be my friend told me where can I buy tickets. And the last one know, in this phrase your father gave an advice to you so instead of verb tell we can use suggest here, so the sentence would come up like " my father suggested me how to save money".

There are always different ways to formate a sentence, even though each of them means the same you need to learn when to use which one. For example, it would be " my sister told me that her secret was", we can also say "my sister told me about her secret which was" it's a bit lengthy but more adequate to say. Moving on to the second one, this one is in passive voice and it should be " a friend of mine told me where can I buy tickets". There were two mistakes, first is when your using mine that means you are describing that this person belongs to you or know you in some way so you don't have to choose my in the starting and the second mistake is if there's a question between your sentence then you use verb first (can) before the subject( I ). Also the active voice of this sentence would be my friend told me where can I buy tickets. And the last one know, in this phrase your father gave an advice to you so instead of verb tell we can use suggest here, so the sentence would come up like " my father suggested me how to save money".