Introductory Spanish is challenging. By the time students reach high school, they are already at a point in their brain development where becoming fluent in a new language promises to be far more of a challenge than it would have been even a couple of years before. However, the first day will go a long way in making Spanish seem like an achievable goal.
Make certain that the students know the rules. A handout is ideal, but this can be accomplished orally. Odds are good that the students will be getting more or less the same speech in each and every class, so focus on how your rules differ from those they are likely to hear in other classrooms.
Have students choose a Spanish name for themselves. Some of them might think themselves above such things, but never underestimate a teenager's ability to have fun. Another reason is so that the student becomes more comfortable with Spanish pronunciation: being forced to say a name like "Leonardo" properly will help that student to remember vowel pronunciation and keep from trying to read Spanish words in English.
Introduce the students to familiar Spanish phrases. Many teachers will start with numbers or letters, which would be useful later, but the first day should introduce the students to the Spanish language. Phrases like "Buenos dias, me llamo Leonardo" and the like, the student will remember tomorrow. "Uno, dos, tres, cuatro y cinco" memorized by rote is less likely to stick.
Have the students introduce themselves using rudimentary Spanish phrases. Not only does it make them slightly more comfortable with their "Spanish" names, but also familiarizes the student with the concept of speaking in a foreign language and makes the language that much less foreign.
Make an introduction into Spanish and Spanish-speaking cultures. This can be accomplished in many ways: a discussion about bullfighting, a lecture about Aztecs, a reading about South American soccer. In addition to (ideally) making the students become interested in the culture, it will create certain touchpoints that will stand out in the mind later: "toronado" is easy to mispronounce as "tornado," but less so when the mind is already comfortable with the correct pronunciation of "toreador."