Audio is one of those things that you have to have for your movie but don't want to really think about. You want it to just magically happen and let's face it, nothing in filmmaking happens "magically". It might look that way but it doesn't. What you need is a good audiophile, somebody that loves to hold a boom with a microphone on the end of it.
You can look for someone talented with sound or you can figure it out yourself. I suggest that you get at least 2 decent remote mics that sync with your camera. While your camera may have sound it is usually really bad. The remote mics can be worn on the body of the actor. These will work adequately but proper mic placement it necessary. When placing the mic on the talent use your fist with thumb extended up and pinkie extended down in the ol' hang loose hand sign. Place the thumb under the chin and at the end of your extended pinkie is where the mic should be placed. This places the mic close enough for good sound but far away enough from the mouth to prevent popping and sibilance.
If you are fortunate to have a mic with a boom then this is the way to go with group scenes. It is a real pain for a boom operator to handle a boom for an entire day of shooting, but those audiophiles just love this stuff. They will hold a boom for days and love it. Once you get your sound back in the studio you will need to edit it along with the image. Previously filmmakers had non-linear systems to edit their sound but now you have available non-linear editing. This means that NLE gives you the ability to move sound clips back and forth within the video itself. Along with recording the dialog of the film there is always sound effects and a soundtrack to provide.
The movie would be quite dry if there is no soundtrack or sound effects. Making a soundtrack can be quite difficult unless it is all original work. This is when you might look for music that is already recorded but that could present quite a few problems. Violating copyright on anything belonging to another artist or their agent that owns the rights to the music, can hold up you up in post-production and keep you from being able to release your film. There are ways around this but only a couple. You can always use music that is public.
Any piece of music or image that has not ownership attached to it means that there is no person or organization that has a proprietary interest in this music or image. The second solution for adding a soundtrack to your film is to simply use original work. This is easy if you happen to have a friend that writes and performs music that works with you film. Many budding composers and musicians are looking for a chance to work with a filmmaker so that they can partner their music making talents.