How to say "I'm finished"? When we've finished doing something like I've finished (doing my homework) what we should say. 
Can we say 好了 "I'm finished"? (Hǎole)

How about "are you finished"?  Can we say "好了吗"?. (Hǎole ma)
Depend on the context, 好 and/or 完 can be used.

好 is more oral friendly as there is no emphasize of any difference between the speaker and the listener. In some cases, using 好 may show a closer relation between the parties of conversation than 完.

On the other hand, 完 is more formal. It is a better parallel to "finish" comparing to 好 as the latter has broader usage. Be weary while using 完 as it is often used when the parties involved in conversion are not equal.

For instance, 完 is used when your boss speaks to you, your teacher speaks to you, your group leader speaks to you, etc. 好 on the other hand, tend to be used when your friend speaks to you, your partner speaks to you, etc.