Is it wo you didi or wo you yí ge didi? I have a younger brother. Do I use yi ge or not? I am confused about the yí ge?
Wǒ yǒu dìdì.
Wǒ yǒu yīgè dìdì.
They're both grammatically correct, and almost the same in meaning. But I feel it would be more normal to say the latter (我有一个弟弟); by itself, the first one sounds a bit incomplete and childish.
Since Chinese doesn't have plural, the first example (strictly speaking) doesn't give a quantity of "younger brothers", whereas the second does. Nevertheless, I expect most people would interpret the first as meaning "one younger brother", otherwise the speaker would have said something else like 我有两个弟弟 (wǒ yǒu liǎng gè dìdì).
Also note the second one is equivalent to:
Wǒ yǒu gè dìdì.
So the 一 (yī) can be omitted too.
Beyond this, it seems more common to say 我有弟弟 within longer sentences (where the sentence doesn't seem as incomplete). E.g.:
Wǒ yǒu dìdì le.
where the 了 is the change-of-state 了, which implies you now have a younger brother, whereas before you didn't. E.g. your mother just gave birth to a son.
Another related example:
Do you have a younger brother?
Nǐ yǒu dìdì ma?
Above, it's probably better to leave off the 一个 since the person might have more than one younger brother.