My puppy won’t listen

My puppy is 8 months old now I got her when she was 6 months, I have been working on training her to sit not jump on people and the basics but she won’t listen and I’m scared her age will get the best of her and I won’t be able to train her anymore because she will be too old, this is my first dog I’ve owned by myself and I’m just struggling, she is very aggressive with every one except my family, she bites people on the ankles if they come onto the property, and me and her are just bucking heads together because she won’t listen and I just think she’s too smart to listen to me, what steps do I need to take to train her correctly, I want her to know I’m her owner and she isn’t top dog around here, please help!!!