George Lucas once said Sound is 50% of your movie... It's that important, if you want to go out and send your movie to Sundance or Cannes then you're going to need to save up some cash or take out a loan... or sell a kidney... to get heard. You're going to want to have a few things to get your audio up to par...
Firstly, A shotgun mic if the foundation of a good audio kit, you can make yourself a boompole by screwing a mic holder into a painters pole or taping the mic to a broom handle (either works, but don't waste cash on a pro boom pole... it's $200-$5000 for a graphite pole!)... A good cheap shotgun mic can be found on ebay... a nice choice is the ATR55 (around 100 bucks if you want some of the neat accessories... I personally use a NTG2 by Rode)

Beware, higher end shotgun mics use XLR cables... and you might need an adapter to plug into a 1/8" stereo mic input. I use the Beachtek adapter that allows two XLR cable inputs at once to get a full stereo feel (it allows higher quality sound editing in post)
Secondly, you'll want a lavalier mic. They're usually wireless but a wired one that has a LONG cable that runs across the room to your camera will work! They usually clip onto your actor's shirt and run to a wireless receiver that's attached to the camera. It allows you to record audio from someone spewing dialogue, especially at wide angles where a shotgun mic would be seen in frame.
The cool thing about wireless receivers for lavalier mics is that you can gets one that will pick up multiple microphones... this is great for when two people are talking in a frame and you need to pick up both of their voices!

Beware lavalier mics are sensitive buggers... turn off your cellphone.
Thirdly, Try not to use a shotgun mic for strict indoor use for it is mainly meant to be used for exterior shoots they pick up a LOT of reflected noise inside. For indoor usage a hypercardioid mic is preferred... my NTG2 is decent in both indoors and outdoors but a seperate indoor mic is preferred.

BUT WAIT!!!!
Your camera doesn't have a mic input!? Oh know! OMGZZZZ! Don't fret. You can pick up a voice recorder, plug in a mic into your laptop or even have one an old tape deck. Be careful though, make sure to record sound for every take and say the scene number and take number in the microphone as well as have an actor clap before you say action to make synching up easier in post.
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Now you've got your sound and you need to link it up on your computer? Go get Audacity...
http://audacity.sourceforge.net/ it's a great FREE program that allows you to run your audio through it and make sure everythings working well, also you can record your own music from your guitar or whatever for some scoring (more on that in a minute)... If you have some cash and you want a great program to really help your audio. Bias Soundsoap is a really user-friendly way to make all of your audio recordings sound smooth, it can clear up fuzzy recordings and even add a layer of compression to your whole movie to make the audio seem unified.
Also adding background noise (like birds chirpping and wind blowing for EXT. shots) will help cover holes in your audio or take attention away from a fuzzy recording.
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Now onto music...
How often have our movies suffered from lack of music? Background music can cover flaws in your audio tracks, it adds a HUGE amount to the storyline, it can build suspense or even relieve the audience... it's has the power to convey emotion and the power to give your film a truly finished look!
Now, to get a full sized orchesta to perform custom pieces that are original and convey the ideas and emotions you want will cost a pretty penny! There are some free sites on the internet that have free to use music and tracks such as
http://www.mobygratis.com/film-music.html,
http://www.royalty-free.tv/ or even
http://www.newgrounds.com/audio/... problem is they probably won't match up very well unless you edit around the music... which in some cases is really hard. (ex. say you have the perfect cut of a suspenceful moment and it took an hour to get every shot cut down and shaved off so it fit exactly right... then you get a song and it completely changes the tempo... sucks right?)... Fruityloops is a free program (demo) that allows you to make some fairly simple work that you can export (can't save in demo mode) and put into your films. I've goofed around in Fruityloops and actually made a silly John Carpenter-esque theme song once.
If anyone wants I use a midi keyboard and gigastudios + reason 4 to make my own orchestal pieces... it's far more elaborate then fruity loops and allows me to have less limitations of sound and functionality.... I was inspired my Robert Rodriguez's Inside Troublemaker Studios (special feature on Once Upon a Time in Mexico) to get myself a midi controller keyboard and score my own movies.
I am willing to make some pieces for you guys, free of charge, for people who want to add the finishing touches on school projects, video festival entries, etc... all I ask is for a name in the credits

... I prefer receiving actual video files to work with so the timing is right plus notes on how it should sound, but if it's for future use I don't mind doing it just with some guidelines.
It'll take sometime if there are multiple requests, but I'll try to keep up especially if deadlines are a matter.
Note I am not a professional musician, so don't expect Fur Elise

I'll do my best to serve the indie community. Tell your friends as well! I also have a electronic drum kit that I will soon be linking up to the computer (waiting for the right cords... damn snail mail) so I can also add more natural sounding drums to the pieces... trying to recreate drums on a keyboard is a bitch.
Well hopefully that covers it! Add any info or suggestions.
- From your friendly neighbourhood Westcroft
UPDATE:
Sorry guys, forgot to mention one of the most important aspect of movie sound... The sound effect *slaps head*.... I was up late writing the post and planned to do a quick overview. so here we go!
Getting sound effects off the internet is a great quick and easy way to get sound effects in your movie. The best free site is
www.sounsnap.com...
My favorite way of getting customized sound effects that fit perfectly in your movies is using the process of foley art... this is recording seperate sounds that you produce and then adding it into your movie. For example tossing a phonebook on the ground for a body hitting the ground and making a thud or layering celery breaking, a clap and some sort of bassy sound to make a nice punching (bone breaking) sound.
If you decide to pick up a shotgun mic or a hypercardioid, they are perfect for recording seperate sounds...