Below are some tips and a possible structure for your first section. In addition to this page, sample first day lesson plans can be found on the First-Day Activities page of the EBRB. Also, feel free to take a look at the plan used yesterday for the first day of this course.
First impressions are important, so it's a good idea to get students on your side early on. For this reason, you might want to do something cool and exciting at the beginning of your first section. Some GSIs like to spend the first five minutes of every section with this sort of attention-getter, but it's essential for your first section. The “show and tell” you choose is totally up to you. You could start with a cool demo, a quote about science, or a recent astronomy news article. If you're willing to go to the effort of getting your hands on a projector, you could show a pretty picture associated with your research and talk about what it means. Spend some time thinking about something cool you'd like to share with your students; this is your chance to show off your creativity and enthusiasm for science!
After your flashy intro, spend a couple of minutes introducing yourself to the students. You might want to tell the students your year in grad school (or undergrad), where you went to college (or high school), where you're from originally, what sub-field within astronomy you like most, etc. Students will feel more comfortable with you if they see you as a person instead of as just an astronomy database!
You and your students should also do some kind of (quick!) icebreaker exercise to learn each other's names (and majors and years in school and hometowns, etc.). You might consider having students make big namecards so they can see each other's names, though the layout of Evans 264 is not especially conducive to being able to see others' namecards.
This all may seem a bit juvenile, but generations of experienced astronomy GSIs have found that these sorts of activities can set you up for success throughout the rest of the semester. The more you can do to develop a friendly, open atmosphere early on, the better.
Go through the syllabus for your section. Also, don't forget to announce TALC, office hours, star parties (if you have any dates yet), homework due dates, test/quiz dates, etc. You should also remember to tell your students what “TALC”, “star parties”, and even “office hours” are!
As mentioned in the GSI Basics, it's important that you do something that allows you to learn your students' names quickly. You should allocate time during your first section to allow you to perform whatever that “something” is: take pictures of your students, ask them to join a Facebook group for your section, write down a chart of names and seats, etc.. (If your “something” doesn't require the participation of the students, you should do it while they're filling out notecards to save time.)
It's important to present an accurate picture of the course as early as possible so after your flashy topic and introduction and syllabus you should conduct a “normal” section. By “normal,” I mean give a brief lecture introduction on the material for that day and then break students into groups to work on a short group worksheet or do a quick demo. Many great first day activities can be found on the First-Day Activities page of the EBRB. Also, as always, feel free to ask your head GSI or AY 300 instructor about some neat first day activities. It helps to make your explicit what things you do will be done every section: “OK, to start I'll recap the week's lectures, which I'll do at the beginning of every section this semester…”
Make sure you enforce section rules, regulations, and norms from the very beginning. If students are not doing what you want them to, correct them the first time it happens on the first day. For example, if you want students to go to the board, make them go to the board; if you let them sit down the first day, you will never get them to stand up later in the semester. Likewise, DO NOT let students go early from the first section! It should be easy for you to find enough stuff to fill 50 minutes in pretty much all of your sections, but it should be especially easy for the first sections of the semester. Thus, this shouldn't be an issue, but I feel obligated to mention it anyway. If you let them go early the first time, they'll come to expect it and that's not good.
Of course, when you're teaching your first section ever, you may not feel like you know exactly what precedents you want to set. Even with all the guidelines and suggestions you've been given, there are many choices to make: Should students choose their groups or should you? How will you go over their worksheet answers? When and how will students write on the board? And so on. The choices that you make regarding questions such as these all set precedents, but as a brand new GSI, you almost certainly don't know what choices will work the best for you and your students.
The best I can say is: that's just how it is. Do whatever makes sense to you and don't be afraid to change your mind. Even after the first section you can make small changes, such as different ways of going over worksheet answers. Plus the next time you teach, you'll have a base of experience to build on, and you can give a better first section. Sadly, there are no shortcuts: teaching, like every other worthwhile activity, just takes time to master.