Sound and image go together like chocolate and peanut butter, so if one is a little off, the whole concoction is spoiled. This helpful video will give you some very practical tips and tricks for creating realistic and effective sound design for your video work.
Coming to you Cinecom, this awesome video will give you some good advice for realistic sound design. You don't need to be an audio specialist in most situations; just having a good understanding of the interplay between sound and video and some basic audio manipulation techniques can help you get through the majority of your work without having to outsource. Of the tips, I think the most important is to properly match the mic distance to that of the video frame. If you have a wide shot of someone cracking an egg into a frying pan and it sounds like it's right next to the viewer's ear, they're going to be jarred out of the world you've created. Don't forget the importance of the room ambiance either. Pure silence is rarely the right audio to use and can be a bit artificial or unsettling, unless that's the effect you're going for, of course.