We use a 'reporting verb' like 'say' or 'tell'. If this verb is in the present tense, it's easy. We just put 'she says' and then the sentence:
Direct speech: I like ice cream.
Reported speech: She says (that) she likes ice cream.
We don't need to change the tense, though probably we do need to change the 'person' from 'I' to 'she', for example. We also may need to change words like 'my' and 'your'.
(As I'm sure you know, often, we can choose if we want to use 'that' or not in English. I've put it in brackets () to show that it's optional. It's the same if you use 'that' or if you don't use 'that'.)
But, if the reporting verb is in the past tense, then we usually change the tenses in the reported speech:
Direct speech: I like ice cream.
Reported speech: She said (that) she liked ice cream.