How do I stop my rim brakes making a squeaking/squealing sound?
V-brakes can be hateful and make a lot of sound if they're improperly adjusted- sometimes they're noisy even when they're properly adjusted, especially on braking surfaces that are not machined. You can usually alleviate this with one or more of the following methods.


First and foremost, make sure your pads are properly adjusted. This is better demonstrated than described, so check out this youtube video to save yourself a lot of reading. If the link goes down, just google v-brake adjustment.
Make sure the braking surface of the rim is clean. Use rubbing alcohol and a green scrub pad to scrub off any crud, and then wipe the braking surface off with a clean towel with a dab of alcohol on it. Using anything besides rubbing alcohol may leave a residue and make the problem worse.
Toe in the brakes. I found this image which not only illustrates what "toeing in" your brakes is, but shows a nifty little trick to set the gap appropriately: 




You may need to toe them in more that what this picture is showing if they continue to squeal on you.