Regardless of what type of pronoun you use, it is important to understand that a pronoun not only replaces a noun, but also all of the description words that go with the noun. To avoid the repetition of long and annoying noun clauses, a pronoun eliminates the noun and all the words that modify (describe) it. Obviously, a pronoun should not be used unless it is clear exactly what the pronoun is replacing. For example:
The extremely beautiful mountain was within sight. It was straight ahead.
In the sample sentence, the pronoun “it” replaces the noun “mountain” and also eliminates all of the modifiers that go with “mountain” because you don't say extremely beautiful “it.” At first you will be replacing simple nouns with pronouns, so there won't be any complex or difficult modifiers. Just remember that when a noun has any modifiers, such as adjectives or articles, they will disappear when that noun becomes a pronoun.