Adobe Spark is relatively easy to use for creating web pages, presentations, newsletters or graphics of any kind. There are dozens of templates to chose from at the start of your project tailored to whatever purpose you might be looking for. Personally however, I find it easiest and most customizable to start from scratch when it comes to making a newsletter that is focused on visual elements rather than text. But if your newsletter is more text-heavy then I recommend choosing from one of the available newsletter templates to save yourself the trouble of formatting. There's not a ton of ability to actually customize beyond what Spark provides, but I think it still works for the newsletter purpose.
Starting from Scratch: Once you have selected your template or "started from scratch," you can play around with the design elements. Spark very clearly guides you through adding a title or header element - you can just drag and drop your image and create a title. For the body, I think that the "glideshow" is a really easy and aesthetically pleasing way to create flow for your newsletter. Simply piece together a few pictures - you can either pull from the Unsplash library (high quality stock photos) or upload your own. You can then adjust the alignment of the elements over your glideshow (text, photos, or videos).
Adding Videos: Anytime you want to include a video for your newsletter, it must be uploaded to Youtube first - Spark only includes videos via the URL.
Adding Photos: There are a few different ways to display pictures in Spark. You can create a gallery of photos, fill the screen, or create a "window." Depending on the context of what you're trying to display, there are different uses for each. I would use the gallery for multiple similar images since Spark only allows you to create one caption for the gallery. I typically use the window as somewhat of a divider between content. It's a nice way to break up material, but it doesn't display the whole image at once (pans across as you scroll down the newsletter) so keep this in mind when you're deciding which method to use to display the image. You can also add a caption to the "window" image that displays in large letters across the image. Probably the most common way to add an image would just be to center it into the newsletter - I prefer this to filling the screen.
Adding Text: Keep it short and sweet. I think the purpose of using Spark rather than MailChimp or other distribution platforms is to focus on the visual element of it. Keep your captions as minimally wordy as you can and be deliberate about the language you choose.