哥哥 (gēge) means 'older brother' and is the term you use for your elder male sibling. 弟弟 (dìdi) is its counterpart for a younger brother; 兄弟 (xiōngdì) often means 'brothers' (both elder and younger) as a plural or collective, but in some dialects can refer to a younger brother. Use 哥哥 when you need to specify the older one.