You can use your camera or a minidisc recorder. The microphone is the first consideration, it is worthwhile considering hiring one (they can be a few pounds a day) if you really want good sound and can record it in a day or two. Make sure the mic you buy is compatible with your recorder.
Whatever sound you want to catch you need it isolated from background noise or "clean", choose the best location for this and get as close to the source as possible whether recording a train or the sound of a kiss.
It may be that the sound you got for that slamming door had someone speaking over it when filming, or it was distorted, maybe it just doesn’t sound like you wanted it to. Go out, get a "clean" version and then paste it over the top of the old sound!
There are music and sound FX libraries available but there may be copyright issues with these, meaning you have to pay to use them. This can be expensive so consider what you are going to use when planning and try and source it yourself or get a free copy! (For more information see chapter 6.)
You may need to go back to a location when things are quieter if there was too much noise during filming. Alternatively you could find a similar sounding location with cleaner sound.
This often means at night! Between 3am and 5am is about the time when traffic and planes are most absent, it’s a tough job but someone has to do it!