Writers---novices or otherwise---tend to pay more attention to the content and its thesis, the explicit reason that any given piece is written. Writers also tend to put thought into developing a style. Yet, without the proper tone, all your research and ingenuity will come to naught. The reader will describe the piece as "clunky," "convoluted" or "directionless"---or worse, the reader may bypass reading it altogether. A piece's tone is the bonding agent, uniting its content and thesis, even its style.
What is "tone of voice" or simply "tone," precisely? As Susan Geye writes in "MiniLessons for Revision": "Tone is a particular way of expressing feelings or attitudes that will influence how the reader feels about the characters, events, and outcomes. Speakers show tone more easily than writers because they can use voice tone, gesture, and facial expressions. A writer must use words alone." In short, tone is a gut-level feeling that writers wish to share with a reader.
Descriptions of tone tend to possess an emotional flair because of its basic nature. Formal examples might include "objective," "ironic" or "dry." However, less formal examples might also include "happy," "dopey," "grumpy" or really any of the seven dwarfs that you can immediately recall; the point being that you hold a one-word descriptor in mind when crafting your piece.
A helpful consideration, which doubles as a warning, is to distinguish carefully between "tone of voice" and "voice" itself, or "style." All writers develop stylistic tics, consciously or not, making their works characteristic of them. A writer might adopt a tone so often that it becomes integral to her style, but think of a musician who knows only one chord; all her songs sound alike. Regardless, tone changes from one piece to the next, depending on subject matter and to an extent, target audience.
When writing, ask yourself, "how do I feel about the subject?" Is this how you want your reader to feel? If the subject matter saddens you and you wish the reader to mourn, then the piece should possess a sad tone. This does not mean that the piece must be devoid of humor or other such elements; each element, however, must indicate on varying levels the sadness of the subject matter. To lose sight of this is to lose sight of your piece's meaning---and the reader certainly won't get it. Say, for instance, the subject is genocide, adopting a chipper tone will irritate and probably alienate readers. Therefore, tone must be modulated accordingly. Tone is not merely the manner of expression on a given subject; it also conveys why you chose to express it at all.
Having an appropriate tone in mind indirectly helps organize the information as you write by giving you greater clarity of purpose. Thus, your finished piece will draw the reader smoothly throughout and perhaps win an admirer.