Ever wonder how people on the television have all those surprise parties? Well, here's how!

[edit] Steps

Decide who will be invited to your party. Make sure that they are all clear that this is a surprise party, and that they keep it hushed.
Make a visit to the store. Buy confetti, cups, plates, napkins, ribbons, and anything else you think is necessary. Tell the "victim" of the surprise party that you are just buying groceries, and be sure that they won't be home when you get back.
Hide all the party supplies in a old trunk, a locked car trunk (with no windows), or someone else's house.
Have everything ready and decorated before the person arrive, this includes having the guests show up!
Think of ideas for how to thoroughly surprise your little "victim". A classic is hiding behind all available furniture and jumping out at the person, but feel free to make edits.

[edit] Tips

Have great music playing
Don't have alchoholic beverages at the party, as people tend to over drink at surprise parties
Make sure there is enough food
Make sure everyone stays happy!
Some great ideas to get the special person out of the house are:
Ask the special person's boy/girl friend to take them out on a date that night and have them be at the house at a certain time.
Get the special person's best friend to take them to a movie, play, or sleepover.
Always make sure your guests arrive earlier than the special person, and that they park in an inconspicuous place (not crowded around your house/apartment, etc.). The reason for this is so that the appearance of many cars won't ruin the surprise early, and you don't risk the chance of the special person recognizing a friend's car. Try having your friends park on a different street or down the block.
If it is for a kid's birthday, have their parents take them to the mall or somewhere as you set up your party.