A predicate is a word that describes the topic/subject of a sentence. The Japanese equivalent of this is a 述語（じゅつご）, which literally means ‘statement word’. As a predicate, な-Adjectives can appear by themselves (when the word they are describing does not need to be stated), or they may be used at the end of a sentence.
It is important to remember that all な-Adjectives can also function as nouns, and therefore tend to sound more like a noun than an adjective, when used as a predicate.

• 素敵（すてき）。
  It's wonderful.
• 彼氏（かれし）は素敵（すてき）です。
  My boyfriend is lovely.

When there are no other words apart from the adjective itself, it comes across similarly to ‘It’s pretty’, or ‘It’s hated’, and similar expressions in English. We just have to guess what the ‘it’ is from context. In many sentences, either は or が could be used to mark the word/words that the predicate is describing. Which one sounds more natural will depend on many factors. In general, if you are specifically pointing out something as being different from other things in the same category, が will be used.

• 有名（ゆうめい）な駅（えき）は汚（きたな）いです。
  Popular stations are dirty. (In general)
• この有名（ゆうめい）な駅（えき）が汚（きたな）いです。
  This popular station is dirty. (Compared to other popular stations)

In this example, が is distinguishing a specific station as having a unique level of dirtiness compared to others. は would sound unnatural in this type of sentence.