First: What do you mean by "above"?
The opposite of above is below, but that does not quite help you to know whether to use arriba, encima, sobre or encima de -- the various ways that the basic concept of the English word above can be expressed in Spanish.
Let's start with sobre. It can mean almost the same as encima de, which conveys the idea of close contact, of something resting on something else. El libro está sobre la mesa (The book is on the table). Sobre can also be used in a non-literal way, as in sobre todo, which means above all. As a noun, sobre is masculine and means an mailing envelope.
Encima de is often used exactly in the same situations as sobre, but with a connotation of a pile of things. More importantly, it can mean that something is hanging above something else. Mi mochila está encima de la ropa (My backpack is on top of the clothes), or Colgamos la ropa encima del patio (We hung the clothes above the deck).
When encima is not used with de, it is an adverb of manner or means. and is best interpreted -- as adverbs are -- by noting its relationship to the verb. Mi amigo se cayó encima de la leña (My friend fell on top of the firewood). This shows the manner in which he fell. Here's another example: Primero, pintó la pared, y encima, puso el sellante (First, he painted the wall, and on top of that, the sealant), or Había mucha nieve, y encima, hielo (There was a lot of snow, and on top of that, ice).
Arriba is also an adverb. It is the most generic translation of above, but as the previous examples show, it may not be the best choice. Use arriba when the sense of "above" is not limited, that is, not so connected with things underneath: Arriba se ven todas las constelaciones del invierno (Above, all the winter constellations can be seen).