The speaker begins the poem with an important instance of personification by calling nature "her." Here, nature—which refers to earth, sky, the organic environment, etc.—becomes a female figure. It even takes on maternal qualities. (This is not to be confused with later uses of "her," which personify the speaker's "little boat" traditionally.)
This initial use of personification is based on the verse paragraph that comes before this passage in The Prelude. There, Wordsworth describes how nature keeps teaching him new lessons. By continuing to refer to nature in such a manner at the start of this passage, Wordsworth suggests that he's about to relate another lesson nature taught him.
Additionally, the phrase "led by her" hints at the kind of relationship the younger speaker has with nature at the outset of his little adventure. Nature, personified as a woman that teaches the speaker lessons, is a mother figure. The speaker has a very intimate and trusting relationship with "her," just as if they were mother and son, members of the same family
Taken out of personified terms: the speaker feels a close attachment to nature. He seems himself as part of the natural environment, continually following its slopes and trails ("led" by it) to discover new things about the world. As the narrative of this passage progresses, though, that relationship changes. Things stop feeling so cozy.
The uses of "her" and "she" that follow refer to the speaker's boat, since boats and ships are traditionally referred to by female pronouns in English. This use also adds a level of grandeur to the boat, since such personification is usually reserved for larger vessels. In the young speaker's imagination, the boat is an "elfin pinnace," an elf's boat. Although to other eyes it might just be a humble rowboat, to the speaker it is imbued with magic.
This continuation of female personification also suggests a connection between nature and the boat, which is supported by the depiction of the boat itself. For instance, the speaker compares the boat to a swan, hinting that it is just as much a part of nature as an animal. And the boat doesn't disturb the water so much as add to its beauty, creating "one track / Of sparkling light." These details suggest that the boat helps strengthen the speaker's connection to nature in the first half of the poem.
The speaker also personifies "mountain echoes" by describing them as a "voice." Mountain echoes could be any noise that filters down from the mountains: wind, birdsong, rustling trees, tumbling rocks, flowing water, etc. By calling them a "voice," though, the speaker again conveys the feeling of intimacy he has with nature, as if the noises of the mountain are talking to him. Some readers might even interpret this moment as verging into the pathetic fallacy (attributing human traits to nonhuman things), which is covered as its own entry of this guide.