It's almost impossible in Russian to predict how a perfective verb is formed. Most dictionaries will list both forms in the same entry.
Some perfectives are formed by adding a prefix to the imperfective form:
imperfective
perfective
делать
сделать
строить
построить
Some aspect pairs are formed by extending the stem of the imperfective form with an infix:
imperfective
perfective
показывать
показать
If one verb of the pair is conjugation I, and the other is conjugation II, most often the conjugation II verb will be perfective:
imperfective
perfective
кончать
кончить
отвечать
ответить
Some aspect pairs are formed from two different stems: