Welcome to the Apple Support Communities
If you only lost GarageBand, iMovie, iPhoto and other iLife apps, you don't have to pay for them anymore because they are free for you. All Macs come with those apps for free.
If your Mac came with DVDs, just insert the Mac OS X Install Disc 2 or the Applications DVD, and install the iLife applications again. Then, open > Software Update, and install the latest version of these applications.
If your Mac didn't come with DVDs or you purchased iMovie, iPhoto and/or GarageBand from the App Store, open App Store > Purchases, and download and install these apps.
In case you lost paid third-party apps, you have to contact with their developers in order to know if you can use the serial number you got to activate them. If you got them from the App Store, you can download them for free in App Store > Purchases