how to backup my apps ?

All apps should be in iTunes on the computer.

App data, if enabled by the developer of the app, is included in the backup of the device done via iTunes or iCloud.

There is no need to back up apps per sé.

App data is backed up to iCloud (ie your favourites in say TomTom or bookmarks in Chrome) but the app itself doesn't need to be backed up. When you restore your phone from backups, your data is restored and the (latest version) of the app is downloaded directly from iTunes in the cloud, so it never needs to be backed up.

That said, if you're super paranoid and want to back up the apps manually, sync with your local iTunes on PC/Mac then look in your iTunes library, you can manually copy the .app files to a backup of your choice. But as I said, you don't really need to. :)

I'm getting a new i-phone 5 and want to keep my apps. I need step by step instructions as to how it's done.

Transferring information from old iPhone to new iPhone