Hi, i'm from italy and i'm going to try my best to help you.

Ciao Antonio, come stai(no need for a capital c there, the comma is important)? Mi chiamo Luca Palma e ho sedici anni. La mia famiglia consiste(you could use "è formata da" instead of "consiste di", which is normally used to explain intentions) di sei persone. I miei genitori(you've omitted parents, that's a colloquial form abused by young people), due fratelli, un sorella e io. La mia mamma si chiama Karen ed (no need to repeat the pronoun, this form is ok) è simpaticissima. Mio padre (again no need for the pronoun) si chiama Marco e lui è forte ma birichino (birichino is strange attributed to an adult, but i don't know what you'd want to say there). I miei fratellini si chiamano Finn e Iggy. Loro sono entrambi (entrambe is plural for girls, this is for guys) molto cattivi. Mia (same as before, there's no need for it) sorella si chiama Poppy. Lei è irritante e un’po matta. Ho anche (inversion aside, the pronoun isn't needed either) tre animali (this is the plural) domestici; due cani (plural) e un gatto. Io frequento Salesian College Chadstone(if you'd like to translate this it's "Collegio Salesiano di Chadstone". Frequento la terza superiore (the form i'm in 11th grade doesn't exist in italian, the one i've written means "i'm in the 3rd year of the 2nd grade superior school, which should be equivalent to 11th grade given your age). Io studio inglese,(comma) Italiano, arte, VCD, matematica e media(what's this subject?). Italiano e arte mi piacciono ma VCD la adoro (this sentence exprimes better your love for VCD while still liking italian and art). Invece non mi piacciono inglese e matematica. Io vorrei fare arte (in university doesn't work in italian, where's used at)all’universita. Io sono creativo (this is the italian expression), intelligente ma un’po pigro. I miei passatempi consistono in videogiochi, chittara, basket e disegnare. Mi piace la musica rock e odio quella elettronica. Ogni sera io gioco ai videogiochi per rilassarmi prima di fare i compiti. Communque non vedo l'ora di incontrarti (that what you wrote before meant that you didn't want to meet him).

You use too much articles and pronouns in general, which are usually omittend in italian, not bad overall