做 and 作 are both verbs.  做 is used when it means " to make something (that is concrete)", e.g. 做衣服, 做文章; 作 is used in a more abstract sense, and more often in written form, and in formal context, (especially in Chinese idioms), e.g. 作罢, 作废, 作文, 装模作样. The difference between these two verbs is sometimes even confusing for a native speaker. The best way to learn them, though it might sound tedious, is to remember which one is used in each word or phrases. Use an authorized dictionary as your guide.