If you don't know how to setup Time Machine in the first place you should see this tutorial first.
Open a new Finder window by clicking its icon in the dock
Select Go to Folder... from the Go Menu.
Input /System/Library/LaunchDaemons/ as the folder to open, then click the Go button.
Locate com.apple.backupd-auto.plist from the list and drag it to your desktop.
Double click the com.apple.backupd-auto.plist file from the Desktop to open it.
Locate the key StartInterval then change its value to the interval of your choice (in seconds).
It will look like this:
Select Save from the File Menu to save your changes.
Now drag the com.apple.backupd-auto.plist from your Desktop to the LaunchDaemons folder.
You will get a message stating that the item "com.apple.backupd-auto.plist' could not be moved because "LaunchDaemons" cannot be modified. Click the Authenicate button.
You will be asked to confirm replacing the existing file with the newer one you are moving. Click the Replace button.
You will now be asked to enter your administrator username and password. Do this then click the OK button.
The next time you reboot your computer the Time Machine backups will take place according to your set interval.