You want your first speaking experience to be a happy one, so I've prepared tips that may help you when writing and presenting your talk.
1. Know your audience
The SeaGL audience, for example, is an interesting mix of those new to tech and folks who have been around technology for a long time. Because the conference is held at a community college, you can expect the Friday audience in particular to have a large number of students.
If the audience will be a mix of experience levels, you can't assume that they'll have the background to understand your talk from the outset. Therefore, consider having a slide or two of introductory information to lay a knowledge foundation and provide context for the less experienced members of your audience. Give them the tools they'll need to not get lost during your talk.
2. Tell a story
You have great content, otherwise your talk wouldn't have been accepted. But how you organize that content makes a big difference to how well it's received.
Take the time to outline your talk before you write the slides. Make sure it tells a coherent story—from beginning to end. Check that each section or slide makes sense and that you can transition to it from the slide or section before. Lack of good transitions can lose your audience and make it a lot harder for you to present the material.
3. Construct your presentation
Don't overload your slides with words or bullet points. The slides are there to guide your performance of the material. They are not there for the audience to read.
When you do have words on your slides, make the font size quite large. Your goal is to make it easily readable from the back of the room, not to fit a lot of text on the slide, so bump that size up.
4. Practice delivering your talk
The single largest and most common mistake new presenters make is not practicing their talk. Believe me, your audience can tell and they don't like it.
Simply stepping through your slides and reading them to yourself is not practicing. It can help you become more familiar with the material, yes, but presenting a good talk requires more than just knowing the material. You also must be comfortable delivering it. You can't gain that comfort without actually delivering the material out loud, just as you will to your audience.
Delivering a talk is a performance, therefore you should practice as you'll perform. Get an audience if you can—a meetup, friends, your family, your pets—but even if you can't, then at least go through the same motions you would if you were presenting before an audience. It really does make a huge difference in your final performance and your audience will appreciate it.
Make eye contact with your audience, allowing your gaze to float around the room and meet the eyes of various people in the room. Don't get stuck looking at only one or two people (even if they're your friends). Keep that gaze moving around.
Keep your posture tall and your stance open. Don't hunch over or hide behind the podium. Your audience will listen to and trust you more if you have that open stance. You won't be hiding anything from them. This stance also makes you appear more confident (no matter how you may feel inside), and audiences react more positively toward confident—but not arrogantly so—speakers.
5. Practice demos
If your presentation includes a live demo, make sure that you practice it just as you practice the rest of your material. Your demo is a part of the performance you'll be giving your audience and deserves the same level of attention as the rest of your content. Whatever form your demo takes (terminal, IDE, web), make sure it's readable by everyone in the audience. Increase all of the font sizes before your talk starts. Fixing readability during your demo is distracting to your audience.
Always have a backup plan in case your demo doesn't work. Perhaps have a video you can play (but don't rely on having audio or internet for it), or a scripted automatic version of the demo that you can fall back to.
It's okay if your demo fails—it happens all the time and audiences are used to it. Having a backup plan, simply not reacting badly to the failure yourself, and in particular spending a lot of time practicing the demo, will ensure that your overall talk is still successful.
6. Consider how to handle Q&A
As the speaker, you get to decide what makes the most sense for handling questions about your talk. Some speakers like it when people ask questions throughout their talk, while others prefer that the audience save all questions for the end. Once in a while a speaker would prefer no questions from the audience at all, instead asking that people come up to the podium after the talk to have a more private conversation.
Your preference may change depending on your talk topic and format, but whichever you pick, please do tell the audience near the start of the talk. This sets up their expectations and allows them to write down their questions or otherwise change their questioning tactics.
Sometimes you end up with an audience member who will not follow your directions and hold their questions to the end, or who otherwise interrupts or disrupts your talk. Their interruptions make you feel uncomfortable, which is bad enough, but they're also ruining the experience for the entire audience, which is unforgivable. As the speaker, it is your responsibility to manage this disruption. Your audience will be grateful if you ask the troublesome individual to please hold their comments for later or to please leave the room. If the offender will not leave the room, ask the room moderator to escort them out.
At all stages of this process, be firm but empathetic and respectful. The disruptor is being a jerk, but you don't improve the situation by being a jerk back to them.
7. Follow up after the talk
Once you've finished delivering your presentation, there are three things you need to do:
1. Share your materials. Do you have slides? Code? Links to projects discussed? Please make sure you share them so people can continue learning from your material. Some conferences ask you to provide your slides to them so they can link them directly to your session in the schedule, but for most conferences you're on your own for posting your materials.
If you don't already have a place where you share these things (and even if you do), I recommend posting them to Internet Archive. The Archive can display PDF slides in an easy-to-read eBook format. If you have videos or audio that accompany your talk, it can display those online, as well. In fact, you can save any type of digital file at Internet Archive. This allows you to collect all of your talk material in one place and make it available freely and for free. My article on how to upload a video to Internet Archive provides instructions for uploading a video to the Archive, but the steps are the same regardless of file type.
2. Propose your talk to other conferences. You've put a lot of time and effort into crafting your talk, so don't just put it on a shelf and never present it again. Improve your return on your time investment by proposing this talk to other conferences. If it was of interest to one audience, it'll be of interest to several. As long as the topic and content remain relevant (and you're not sick of delivering it), there'll be an audience for it.
3. Seek out feedback. Whether you choose to propose your talk again or not, seek out feedback both about the content and about your delivery of it. Some conferences provide feedback and talk rating systems, which can be very helpful. Whether they do or not, take the time to talk to your audience members and ask them how they liked it and how you could improve the presentation in the future. Your public speaking will improve much more quickly with feedback than without.
These tips are only for the most common problems and oversights of new speakers. For more information, check out this two hour speaker training workshop and this GitHub repository of public speaking resources.