All of these questions can help determine what type of video you should make and where you should post it. For example, if your target audience is not familiar with your company, you probably want to make a video that focuses on brand awareness before producing an in-depth, product video. You'll also want to host your video on a site that already has a large reach, like YouTube.
Opinions vary greatly among sound engineers on the best method and equipment for recording audio with a DSLR. You've likely seen many videos that use a lavalier microphone — the small piece that clips below the collar of the talent's shirt. Lavaliers come in both wired and wireless options. However, lavaliers can be a bit obtrusive both for the talent (who has to have a wire threaded down his or her shirt) and for the viewer (who has to see a microphone for the whole video).
Sadly, posting a video on an otherwise blank page won’t do much for your SEO. Make sure you embed your video on a page full of relevant and informative content. This will let search engine crawlers know you have not only engaging multimedia on your site, but that you provide a ton of valuable and useful information. Be sure the content relates to your video — sometimes, posting the video transcript can help a lot, too.