You should say: братан, чувак, дружище, приятель

I would like to suggest one more translation for this word that has not been mentioned in answers of other users here. You can also say "старик" but it is very informal and usage of this word strongly depends on the context and situation! It might be used in conversation with a friend who you know for ages. You must not use it in formal situations. In this context word "старик" does not mean "old man"(which is its main meaning) but "old friend". However, many other translations like: братан, братуха, чувак, кент are also informal and even slang. Приятель, друг are more stylistically neutral. Дружище is informal but not slang I think.

To a girl you may say "подруга" But actually, a word "bro" has become an international one, it's often used in modern slang and pictures

I would say дружище. It sounds really nice and friendly. For example, - Привет, Дружище! Как поживаешь? You can also hear "браток" from time to time but it's more common between men. I wouldn't say that to a male friend. Hope this helps.

There are very many ways to say it: брат, братела, братуха, чувак, друг, дружище... You can also just say "Bro" (Бро), in this case we can use it too.