You usually can write it before and after. There is not a big difference. Before - It explain something about the noun. After - It specified something about the noun. But you know... about the meaning... everybody is going to understand what are you talking about no matter where you write it, it's just that sometimes it could sound strange for a native speaker and in some situations it could be ambiguous if you want to specify and write it before or vice versa.

It is true that the adjective can go before or after the noun. If you are just learning, I recommend that you use adjectives after nouns. For example "Es un libro entretenido". Sounds correct. However "Es un entretenido libro" does not sound natural. Another example: "Es un día lluvioso" is OK. But "Es un lluvioso día" again sounds unnatural.