Is it wo you didi or wo you yí ge didi?
我有弟弟。
Wǒ yǒu dìdì.
我有一个弟弟。
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 &quot;younger brothers&quot;, whereas the second does.  Nevertheless, I expect most people would interpret the first as meaning &quot;one younger brother&quot;, 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:

我有个弟弟。
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.:

我有弟弟了。
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?
你有弟弟吗？
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.