Hello Shaique, there is no difference. I've simple means I have. Therefore in both sentences you are saying the same thing. That you have just finished your homework.

If you say you "just" did something, then you need to use the present perfect, like with using "yet" or "already." If you say "I just finished my homework" it's grammatically incorrect, but this is something that most native speakers get wrong also, so it's not a big deal.

Ahh thank you guys It's really helpful :)

They are basically the same thing. “I just finished my homework” is more casual/conversational and “I’ve just finished...” sounds a bit more formal compared to the other option.