Tutor profile: Nicholas R.
What Spanish should I choose since the pronunciations are different? Spanish from Spain or Latin America?
You should make the choice yourself. However, if you are going to be interacting with lots of people from Latin America, it's preferable that you learn Latin American Spanish. But if you are in Europe, you should stick with Spanish from Spain where it will be more useful However, the choice is yours even though you will still be able to understand the other no matter the dialect.
How are "être" and "avoir" different when used in the present perfect tense?
Etre and avoir are quite different. Etre is used in the present perfect tense with (DR MRS VANDERTRAMP verbs) Devenir, Revenir, Monter, Rester, Sortir, Venir, Aller, Naître, Descendre, Entrer, Rentrer, Tomber, Retourner, Arriver, Mourir, Partir, Passer while avoir is used with all other verbs.
Why is it that after hours of studying English and speaking it, I still do not sound like a native speaker?
This is mostly because you have not found out about connected speech yet. When you learn to link words and improve your intonation, you will be well on your way to sounding more natural and native.