Zovem se Justin

Zdravo,
zovem se Justin i ja sam Amerikanac. Ja živim u Midvilu.

Justin, please capitalize the name of a nation. 

K.

Zovem se Justin
Zdravo,
zovem se Justin i ja sam amerikanac. Ja živim u Midvilu.

________________________________________________

Zdravo, 

Ime mi je Džastin (još bolje: Ja sam Džastin)

Amerikanac sam. Živim u Midvilu

OBJAŠNJENJA:

"Zovem se Justin" - Stilska greška je u činjenici da se izbegava upotreba konstukcije "Zovem se..." jer u doslovnom smislu to bi značilo sledeće: "Ja sebe zovem (kako bih se čuo, dozvao, javio...)"

"Justin" - Imena u srpskom jeziku se pišu onako kako se izgovarjau, u vašem slučaju verovatno se izgovara kao "Džastin".

"Ja sam amerikanac" - Imenice koje označavaju pripadnost naciji pišu se sa početnim velikim slovom (Amerikanac, Srbin, Englez, Rus, Nemac...) Drugačije je sa pripadnošću verispovesti (hrišćanin, katolik, pravoslavac, musliman), političkoj stranci (republikanac, demokrata)...

Zovem se Justin

Zdravo,
zovem se Justin i ja sam amerikanac. Ja živim u Midvilu.

Zovem se Džastin (in Serbian language the  names are written as they are pronounced,but you can leave it as you would normaly write your name in English, although the first option is preferable. If you write any name in cirilic you'll have to write it as it is pronounced but if you write it in latin you can write it both ways).

Ja sam Amerikanac. (We capitalize the names of nations but the adjectives derived from it are written in lower case.)

Zovem se Justin.

Zdravo, zovem se Justin i ja sam a Amerikanac. Ja živim u Midvilu.

Justin, please capitalize the name of a nation.

Also, it is not necessary to repeat so many times this "Ja". Just like English speakers, we also almost always try to escape the personal pronounce if it is possible. 

So the best way would be:

" Zdravo, zovem se Justin. Ja sam Amerikanac i živim u Midvilu. "

Good job, keep it up.

If you need any help more, please feel free to contact me.  

Zovem se Justin (Džastin)

Zdravo,
zovem se Justin (Džastin). Ja sam Amerikanac i živim u Midvilu.

Hello, Justin. In Serbian, it is better to write the names as they are heard. But it is not that big of a mistake not to. That's why I didn't write that as a mistake. The names of nations are capitalized in Serbian, but the names of languages are not (so it would be "engleski" for English). As you can see, there are no any major grammar errors, just a slight rewrite (as I would say it in a daily spontaneous conversation). Nice job! Just continue doing what you've been doing so far! Bye!