Moja stan u Hrvatski

Moja obitelj ima jedan stan u Hrvatskoj. - My family has an apartment in Croatia. (its u prep.)
Stan je u Novigradu. - The apartment is in Novigrad. (you have to use je u Novigradu or nalazi se u Novigradu, you can't mix both; also you have the wrong case of Novigrad)
Novigrad je jedan grad u Istri. - Novigrad is a city in Istria.( Again wrong case.; We don't say Istria that is in english, the nominative is Istra and the dative is Istri, u is the prep. that goes with dative)
Istra je u sjevernoj Hrvatskoj. Istria is in the north of Croatia. (again wrong cases; it would be better to use nalazi se instead of Istra je)
To je pet sto metara na moru. It is fivehundred meters to the sea. ( I do not understamd the english sentence)
Sjetim na moru svaka danas. I walk to the sea every day.(Šećem do mora svaki dan.)

Moj (1) a stan u Hrvatskoj (2) i

1. Stan - flat --- masculine

2. u HrvatskOJ, u EngleskOJ, u ŠkotskOJ, u ŠvedskOJ, u NjemačkOJ, u DanskOJ, u ŠvicarskOJ, etc

So, if the country finishes on SKA or ČKA, we need to change it to A to OJ. 

However, we say: 

u SrbijI, u BosnI, u ItalijI, u ŠpanijI, u ItalijI, u RusijI, u BelorusijI, etc

3. You cannot go "on the top of the country" to use the preposition "na - on". That is why we need "u - in". 

4. Nalaziti se --- to be located (one of the reflexive verbs more)

5. You are just giving some information that city. That is why we need a nominative case.

6. Locative case

7. Genitive case

Ok, keep it up. 

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

Have a nice day. 

Moja stan u Hrvatski

Moja obitelj ima jedan stan u Hrvatskoj. - My family has an apartment in Croatia. (it's superfluous to say "one" because we distinguish the number from the word form)
Stan se nalazi u Novigradu. - The apartment is in Novigrad. ("je" is a wrong person)
Novigrad je jedan gradu u Istria. - Novigrad is a city in Istria.
Istraia je u sjevernoja Hravatskoji. Istria is in the north of Croatia. (adjectives should match the case of the noun).
Stan (to) je petsto metara odna morua. It is fivehundred meters to the sea. (the apartment is [located] 500 meters from the sea, not 500 meters on the sea.
Sjetim na moru svaka danas Šetam do mora svakog dana. I walk to the sea every day. (Your sentence might be understood as "I think/sit on the sea every day". Sjetim is a form of "remember", but close to "sjedim" which is I am sitting).

Moj stan u Hrvatskoj (Moj because STAN is masculine noun so the adjective must be too, Hrvatskoj is the locative case of the noun Hrvatska, the problem is that the declansion of the nouns in -ska is the same as of the adjectives ATTENTION: ITALIJA-U ITALIJI, the ending is I, but HRVATSKA -U HRVATSKOJ, the ending is OJ, like I said because it is like with the adjectives )

Moja obitelj ima jedan stan u (the prepositions with countries is always IN) Hrvatskoj. - My family has an apartment in Croatia.
Stan se nalazi u Novigradu. (the verb is nalaziti se, it so called reflexive verb that always has the pronoun SE. Like ON SE ZOVE, MI SE SMEJEMO etc. Novigrad must be in locative, the ending is U for the masculine nouns that end in consonant) - The apartment is in Novigrad.
Novigrad je jedan grad u Istri. (grad is the part of the predicate and it refers to the subject so it must be in the same case, which is nominative here, our word for Istria, is Istra and here you are supposed to use the locative case)- Novigrad is a city in Istria.
Istra je u sjevernoj Hravtskoj.(As I explained above, the ending for adjectives in locative is OJ, Hrvatska, Grčka, Poljska, Švedska are all nouns but with the adjectival origin so zou declain them as thez were adjectives)Istria is in the north of Croatia.
To (what does it stand for?) je petsto metara od mora. (I believe that the meaning should be that the appartment is 500 meters away from the sea. The preposition is always OD followed by the genitive case) It is fivehundred meters to the sea.
Šetam do mora svaki dan/svakog dana. (Sjetim means I remember, Šetam means I walk, na moru would be on the surface of the sea, do mora means to the sea, the preposition is DO followed by the genitive case, danas means today, svaki dan/svakog dana is the same, one is accusative, the other is genitive used to express time, like with SPAVAM CELE NOĆI/CELU NOĆ-I SLEEP WHOLE NIGHT)I walk to the sea every day.